At the moment I'm more of a buyer than a seller, but seeing as my current project is rebuilding my wardrobe with a more stylish and somewhat dressier look, the online second-hand sources I frequent have been absolute gold. My current order contains 4 trousers and one silk shirt, the total cost of which, including shipping, will be about half the price of one new pair of jeans, with a 14 day return policy for anything that doesn't fit.
I have a co-worker who's really into the whole buying-reselling thing. Her specialty is porcelain and tableware of various kinds. Just the other day she sold a (rather ugly) decorative lamp at an online auktion for £370. I can't recall what she paid for it, but the profit marigin was several hundred percent.
I wouldn't mind getting into it, it seems both fun and profitable. I'm guessing having a niche pays of, so you know what to look for in detail. It's the sort of thing I'd imagine aspies would have an edge in. Then again, my niche would probably be knives and sharp objects, and I'd probably end up keeping them all...
As for mean, I don't see it. I mean, it would be mean to track down the original owner and taunt them with the profit you made or something, but it's not like you cheat them by paying their asking price for something you suspect you can re-sell for more. Granted, in some cases it's less of a gamble than in others, but still...
Apologies to OP for OT.
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I'm bored out of my skull, let's play a different game. Let's pay a visit down below and cast the world in flame.