there are certain categories of things that almost everyone seems to want...all the things I'm interested in, like antique tools, chainsaws, etc!
When something kinda interesting shows up that I want, there will always be a couple of guys out there who want it, and they will have it, period. I've even fallen prey to that myself, and gotten raped at least once. OTOH, we couldn't hardly give away my wife's old sewing machine when she got a new one.
I would suggest watching ebay carefully and learning how it all goes before jumping in. For example, my first Ebay saw was an old Homie that cost more to ship than I paid for the saw...I learned to ask about shipping first. I work with a guy that is heavily into silver art pieces, etc. and ebays a lot...he unabashedly admits that he makes good money from "handling" fees!
There are a lot of reasons to go there...I sometimes do just for entertainment. But I'm not enough of a wheeler-dealer to make ot work out for me.