I'll start with a simple one.
In some categories such as year-old models of cameras, look for the one where the seller has photographed the camera (or whatever) on his own dinner table or carpet and posts his own unique photo. Beware the opposite, the Power Sellers unloading hundreds of customer returns of the same model. Read the description and appraise if the seller is really unloading it to buy the next year's model like he said he is. A seldom used camera from a fanatical enthusiast is likely to be as-new with all the manuals and accessories, and the price will be better, than a customer return from a Power Seller that is missing the charger or lens cover etc. Exchange some emails with the seller and size up if he is sincere.
I'll let someone else describe how to read feedback and identify good quality sellers. That is another skill you need to learn before bidding more than say $20 on ebay.
After a while you develop an intuition. Over on TBN today I said I identified a bogus auction as soon as it was listed and MrJimi, of course, was the first to argue with me. I was right.
Massey MF 240 on Ebay - Scam?
Here's another pointer for the first timer. Do some serious searching in Completed Auctions to learn features and prices then never bid more than the average recent winning price. Also search Google, Altavista, etc for price comparisons. Soemtimes ebay prices are higher than retail for some reason.
And never bid until right at the end of the auction. The last thing you want is a price competition as the days go by. When no interest is revealed, other bidders are more likely to bid low. Bid once, a fair price that *you* can live with if you have to pay that much, and then hope you won. Also, with a little experience you can watch with two windows open and bid in the last moment, so no one has an opportunity to counter your bid.
My background: Joined Ebay 5/1998, feedback 230 all positive. I was a Power Seller during 2001-2002 when I sold about $15,000 of network backup tape drives, with the rest of my transactions just occasional hobby stuff. Never been scammed.