The only people I have issues with are the so-called collectors who buy every figure on a store peg wall and sell them ALL on eBay or scalp them in person. If you're selling just one or a few of the stash you've purchased for your own collection and pleasure than that's fine. Lots of people trade and sell their stuff to try to turn a small profit or get something else that they don't own or might even be cooler.
It's the plunder-and-pillage store scalpers who buy fifteen figures, ships and/or vehicles and then put them all on eBay or on a table at a flea market for double or triple the retail price that piss me off. A few here and there isn't objectionable. Doing it regularly and with almost everything you buy is. If you're not supporting, feeding and clothing your family by scalping then you have no business doing it, but even then it comes off as shady and dirty.