Hey Anon - thanks very much for your post and yes it is hard work selling anything online. But it can also be extremely rewarding. Quite a few people in my family make a few dollars here and there selling stuff on etsy.
Anyway, to answer your question, yes some people have a fetish for used underwear. It's not something I'm into - when I buy clothes I like them to be brand spanking new and still in their packets. I even have hangups about buying swimwear that doesn't have that protective strip on.
But like most fetishes it is hard to see the attraction if you don't have this particular fetish.
A few years ago I visited Japan and as a fan of the seedy side of human nature I went in search of the rumoured vending machines that sold used school girl panties. I have to say that it was hard to find such a thing, but I did find a few adult stores that sold used idol underwear. So I guess the rumours were kind of true.
I know eBay have banned listings of used underwear and I believe in some countries you can get in trouble with the authorities on the technicality of sending soiled materials through the post. The practice seems to have moved underground but you might still see such listings on Craigs List or Reddit.
As to the practice itself, well it's a nice way for minor celebs and idols to make some money from connecting with their fans, so I won't condone the practice.
Why do people want used underwear? Personally I don't think this is too much different to collectors spending vast sums of money on clothes once worn by Marilyn Monroe, Princess Diana and other famous celebs.