- the products -- watch for a while and make sure they are selling things you want. That's things, plural. Don't buy a $50 bid pack, win or lose one auction, and then realize you have no further use for those bids. And very few sites offer refunds on unused bids.
- the prices -- on sites with lots of users and few auctions, you won't get much of a deal.
- the reputation -- on a site with few users and lots of auctions, you ought to get a better deal, but ask yourself, if the deal is so great, where is everybody? Before putting down your hard-earned cash, scout around the Internet a bit and see what other people think of the site.
- the extras -- check out shipping-and-handling charges (which can take a big chunk out of the savings), the help section, the return policy. Be smart.
I encourage anybody with a story, good or bad, about a specific site, to put it in the comments.