Because why not, maybe someone will buy it from you as they don’t know better, and you cash in the profit. It doesn’t cost anything to post something on eBay after all.
Some of them don’t even have the thing in the first place, if someone buys the listing they go get one from Amazon or a local shop and send it.
If you actually want to know what people are willing to pay for something though, look at the completed auctions.
The first Fallout barely had anything, but Fallout 2 sure is something else having specific perks, traits and statistics about sex, a bunch of people you can have sex with and some you can get pregnant, and you can work as a prostitute or a porn actor. Or force some of your companions to do so.