Buying on ebay if you don't know what the items are or what they are worth is kind of dangerous. I look at the item, determine its worth by a percentage of what it would cost to get new. If it is in new shape, perfect, no problems and the buyer has good feedback, maybe 75% but make sure shipping is included in that. If you have to pay more, get new. If it is not new, shows wear, anything wrong, I adjust it to where I feel I can be ok in it. It is hard to put a fixed number on it because every case is different. You need to be really carefull if you don't know the market really well. I have been in the business all my life so it makes it easier. Some people are not cautious at all and end up paying almost retail for junk. Watch that and don't get in a bidding war with them. You will end up paying too much. Figure out your top dollar you want to spend, use a snipe program and have it place your bid for you. That aleviates jacking the bid up by sellers friends and people that don't know what they are doing. Have it place your bid at about 10 seconds to the end of the auction.

Research ended auctions for the same item to help figure what it will sell for, and also all the other web based sales classifieds and anything else you can find.

Good luck. It can be a fun game. Just watch out for the guys with no feedback. Some of them are going to hose you. The only guy that got me was a 0 feedback seller who didn't include pictures and I didn't see the item till the end. Ufortunately, I got taken on that one, but over 500 other transactions have been great.

Do the research. It will save your tail!