Most are not worth much, and that includes almost every clear-glass lamp in an attic. The exceptions are specific enough to check in a few minutes.
Plain clear-glass hurricane and chamber lamps were made in enormous numbers through the twentieth century and are still made now. Supply is high, condition is usually mediocre, and a working replacement costs about twenty dollars new. That combination puts most of them below the price of a new one.
That includes our own lamps, and we would rather say so than pretend otherwise. Everything we build is a current product you can order boxed today, so a second-hand one is competing with a new one and losing.
A named maker. Nineteenth-century American and European manufacturers whose names appear in collector references are where the money is. The name is usually on the burner or moulded into the base.
Colored or cased glass. Cranberry, cobalt, milk glass, opalescent and cased pieces sit in a different market from clear glass. Color is the single most visible value signal on a lamp.
A matching shade. Shades broke first and were replaced with whatever fitted. An original shade that matches the font can be worth more than the lamp under it, and a mismatched one drags the whole piece down.
Original burner and chimney. Correct period hardware, still marked, still working. Replacements are obvious to anyone buying seriously.
Condition that has not been improved. Repainted brass, polished-through plating and glued glass all cost more value than the flaw they hide.
Work through it in order. Is there a maker's name anywhere on the burner, collar or base? Is the glass anything other than clear? Is there a shade, and does it look like it belongs? Are the burner and chimney period-correct, or modern replacements?
Four noes means an ordinary utility lamp. It may be a good lamp, and it will still light a room during an outage, but it is not an investment. One or two yeses is worth having looked at properly.
Sold listings, not asking prices. Auction sites let you filter to completed sales, and that filter is the difference between what people hope for and what anyone paid. Asking prices on antique lamps are notoriously optimistic, and a lamp listed at four hundred dollars for two years is a lamp worth rather less than four hundred dollars.
For anything that looks genuinely period, a local auction house or an appraiser who handles decorative glass will tell you more in ten minutes than a week of comparing photographs.
There is a separate argument for keeping one that has no resale value at all. It works without power, the fuel stores indefinitely, and parts for the common sizes are still made. A lamp worth fifteen dollars on the open market is worth considerably more than that on the third night of an outage.
If that is the plan, check the burner and wick before you store it rather than during the emergency. We list the common sizes in our parts finder, and we still make wicks in most of them.
Photograph the markings, not just the lamp. Buyers who pay well are buying the maker's name and the glass, and they will scroll past a listing that shows neither. Say plainly which parts are original and which are replacements. And weigh shipping honestly: packing a glass lamp so it survives the post costs enough to erase the profit on a cheap one.