It depends.

eBay reflects the true market price, so it's a good place to sell. It's also a great alternative to *zon if you don't like Bezos.

craigslist is a great place to find bargains, but selection is a lot smaller, and you have constantly scan and respond quickly for the good stuff.

shopgoodwill.com, and also many local/regional goodwills have their own websites.

They're a great resource to find bargains, as well as a good variety of obscure stuff, without much competition, but once again, selection is limited, and there are bidding wars.

tcgplayer.com for cards 'n shit.