Estella gave you some good info. However, I'll add a couple of things based on my experience with online repo car auctions. They can be a great place to find bargains if you know what you're doing.
1- Look for the kind of car you're looking for and study what's been going on: how much they've been going for on average, how much offer there is, etc. This is much easier to do online than offline.
2- Check each seller's history. Actually get in touch with them and with previous buyers ( if the seller is legit, he won't have a problem putting you in contact with another satisfied customer ). Ask all the questions you feel necessary. Read other people's questions, too.
3- Check the car's VIN yourself ( if it's not older than '81 ) and get a full vehicle history report ( I like Autocheck for this). This is very important and can be done online. Double check all data provided by the seller and verify you have all the info you need to make an informed decision.
4- If the car is not in your state, do be careful and check what the final price will be after delivery. Generally speaking, take into consideration any extra expenses before you determine what your top bid is going to be.
5-Check for auctions with misspellings or mistakes of some sort made by the seller when listing the item ( this does happen more frequently than you'd think ). Bids will be tipically less in these cases, and you can get real bargains this way.
5-You don't need to check hundreds of sites, just go to the most respected ones, like ebay motors. Stay away from Yahoo auctions. I've seen too many scams there. In any case, always be diligent with your inquiries.
Here's a site with updated listings on repossessed cars alone on ebay. Also, here's an article with tips on repossessed cars for sale .