You can find a lot of strange things on Craigslist, but one of the most noticeable fields on that website is the fact that there are a lot of people selling used underwear. It might sound gross to some people, but there are a lot of reasons to go on the Internet to buy a used bra or used panties. Not all of them are as deviant as you might think.
There are any number of reasons to sell used underwear, but they all boil down to one, essential fact; people will pay for it. It might be Internet models catering to a fetish (used unwashed panties in particular seem to be something that a lot of people enjoy, sexually), or it might be women who have a unique and expensive undergarmeant like a corset that they just don't wear anymore or have outgrown. There's profit to be made selling used underwear, and as such there are a lot of women, and admittedly some men too, who participate in that unique opportunity.
There are entities, such as The Bra Museum, who make it a point to collect vintage undergarmeants. Independent collectors may also seek out used undergarmeants that are particularly rare or historically viable. Some people might see a brand name or exotic piece of lingerie that's never been worn, or which has been worn rarely, and decide they have to own it. Then there are individuals who buy used underwear because that's their preference.
Is it really a good idea to buy used lingerie? In some circumstances, such as a particularly low price, a vintage item, or a brand name someone wouldn't be able to otherwise afford, then it's a good buy. On the other hand that's the exception that proves the rule. Nine times out of ten it's a better idea to buy new underwear. Used underwear, even used underwear that was well-taken care of, will have a shorter life span than new underwear will. There's also the questions about what the previous owner got up to while wearing it, which is a particularly niggling question if one is buying used lingerie that's particularly sexy. Can it ever be clean enough for you to feel really comfortable while you're wearing it? For most people the answer is no, and that discomfort alone makes the purchase not worth it.