So, let's say you just found an old light bulb, and you want to know how much it's worth. I'll presume you want to put it up for sale, probably on an auction website like eBay.com (and you want to have all the right buyers see it and have a good sale and all that). This is actually a fairly hard thing to figure out, as their value is based only on what a very small pool of buyers are willing to pay for them, and even the amount they are willing to pay varies over time. (there's not that many light bulb collectors out there). But, I've written this guide to hopefully help you figure out where your light bulb stands. Please use this guide as a general guide only, and not as a set of rules. The only rule to the value of antiques is that there is no rule. They're only worth whatever a buyer wants to pay for them. Supply varies wildly depending on the kind of bulb, and demand has a heavy tendency to concentrate on bulbs that are low in supply. If you desire, you may contact me for additional help.
But first, you need to know this: Lately (2019) I have noticed a disturbing trend on eBay.com, where so many sellers are putting up light bulbs with totally arbitrary prices, the vast majority of which are totally unrealistic and a complete fantasy. I have seen many light bulbs being offered with a buy-it-now price that is 2x, 3x, 5x, even 10x the real value of the bulb. So they just sit there, getting relisted over and over for the same insane price, forever, until the seller finally gives up and reduces the price dramatically. I don't know why this is happening, but it is a real epidemic. Nobody is going to buy a $5-$10 Varicure 260w carbon heat bulb for $139.90, or a $10 50 watt Edison Mazda bulb for $59.99 (plus shipping!), or a $5 (if that) 1000W large GE incandescent bulb for $80... all of which are extremely common finds and are a tough sell even in perfect condition. Even the rarest versions of these bulbs (such as those made in colored glass for example) aren't worth anywhere near as much as that. After making various offers for several bulbs (through the "Make an offer" option), I did notice that whenever a bulb is sold for less than its asking price, if you look again at the closed listing it will say that the bulb was sold at full price, even if a much lower price was accepted by the seller. Obviously this is confusing people and making them think that antique bulbs are worth far more than they're really worth. It is a problem within eBay itself, and it's wasting everyone's time. So, just be aware of this. Now, to the rest of the article...
In general, it's best to start with the age of the light bulb, then look at its condition, then look at its overall appeal to a collector or antiques enthusiast.
The thing that will affect the value of your old light bulb the most is whether or not it is still able to do what it was originally made to do. (Here's a tutorial on how to test a light bulb without damaging it. This is especially important for low voltage bulbs, of which many exist). Does it still work? Is it in good shape? Is the filament intact or broken? If the light bulb no longer works, in most cases it will be worth nothing, or very little, although this depends on what kind of bulb it is (there are a few rare exceptions). If your antique light bulb is in absolutely perfect condition: Working, no glass darkening, no scratches, intact tip, no corrosion, and with an intact label... then clearly you have a winner. But if the glass is darkened from use, or the glass envelope is visibly scratched, or the tip is chipped or broken, or the base is heavily rusted or damaged, or it is missing all labels or any other identifying info, or worse, it doesn't work, then there could be a problem finding a buyer for it. Keep in mind that a bulb from the 1890's in bad shape can still be worth more than a bulb in perfect condition from say, the 1940's, so you should not assume that because a light bulb is in great shape, that someone would buy it. Certain kinds of old light bulbs are extremely abundant, many of them in perfect condition, so don't just factor in the condition.
It can be safe to say that all bulbs made before the year 1900 will sell at an online auction, working or not (assuming they are at least in good cosmetic shape). These bulbs are very scarce and coveted by collectors. How can you tell if a bulb was made before 1900? A good way is to look at the bulb's base type. Common 19th century bulbs have Westinghouse Pin Bases, Thomson-Houston bases, or early Edison bases, all three insulated either with plaster or with porcelain. Although there are a few exceptions (Westinghouse-based bulbs with black glass insulation do exist), the most common pre-1900's bulbs have porcelain insulation on the bottom. A common bulb from that era in absolutely perfect condition can sell for hundreds, while a non-working one in fair shape might sell for up to around $50, in my experience. Though in the case of most Thomson-Houston based bulbs, they don't seem to sell well if they do not work, as they are not so uncommon. I have bought a few of them before for around $30 in working condition. A non-working Thomson-Houston bulb might take a few tries to be sold successfully on eBay.
Rare antique light bulbs are often those that are well known and that were produced in very small numbers, and/or few of them have survived their journey through time. ("antique" generally assumes an age of at least 100 years). A lot of people seem to think that just because they found a bulb that was produced in a small number, or has a rare feature or label, it is worth a lot of money. This is not usually the case. If the bulb in question was not significant in the evolution of lighting history, or is not otherwise well-known for anything in particular, it probably isn't worth much to a collector, and in that case is probably more interesting to a commercial antiques dealer. I've seen bulbs with very unique labels that didn't receive any interest because the seller priced them very high, thinking they were very valuable just because of the unusual label.
Below you can find pictures of early light bulb bases. Note that there are a lot of other early base types not shown here. The ones shown below range from "spotted for sale somewhat often" and "very rarely seen", but not so rare that they're never seen. I would call the other base types "Museum grade" types, since the only reliable way to see them is probably only at a museum. Though a few elite private collectors have bulbs with such bases (I'm not one of them, I'm more of a budget collector). In the early days of electric lighting, you could place an order for light bulbs and ask for them to be made with a specific base type, right from the catalog.
Early Thomson-Houston base, with plaster or red fiber insulation. Bulbs with this base and in good working condition are quite rare and sought after.
Brass Thomson-Houston base, with porcelain insulation. The most common non-Edison base type. Bulbs with this base are a somewhat common find.
Westinghouse Pin Base. Used mostly during the 1890's. Apparently the 2nd most common non-Edison base after the Thomson-Houston base. The example shown has porcelain insulation.
Early Edison base, with plaster insulation and button contact. Used during the late 1880's and 1890's only. Replaced in 1900 by an improved version with porcelain insulation.
Porcelain-insulated Edison base. Used only for a brief time around the year 1900. It was replaced with the Edison base we know today, with black glass insulation.
Westinghouse-Edison combo base. Extremely scarce, shown with porcelain insulation. Will fit into Edison and Westinghouse clip-on sockets.
Below you will find two of the most common antique light bulbs. Bulbs like these in excellent working condition commonly sell for about $5-$15 each, but are usually worth nothing in non-working condition or heavily-used condition.
Mazda light bulb, circa 1918. The clear 40 watt and 50 watt versions are extremely abundant. They use an uncoiled tungsten filament, usually called a "cage" filament. Tipless versions also exist and are from circa 1922-1926. In recent years, replicas of these bulbs have been mass-produced.
Carbon light bulb, circa 1909. These bulbs use a filament made of carbon, which consumes about 3 times more energy than tungsten to produce the same light. (Note: the bulb shown could be a GEM bulb, which are slightly less common).