1.  Do not follow e-mail addresses that are included in the text section of postings.  If the seller claims that is the only way to contact them, abandon ship and go no further.  Send all the questions you have ONLY via the Ebay mail link (Ask Seller A Question).  When you do send a message, ask about payment methods and shipping and such, not just about the item itself.  If you don't like the options the seller gives for payment method, tell him what you would be willing to do.  If he is real, he may try to accomodate you because he has the item and he wants to sell it.

2.  Short duration sales may be suspicious.  There are a few good reasons to sell quickly (such as a high-demand commodity item where there are many sellers and many items), but it may also be setting the stage for a hit-and-run seller.  Scammers want to quickly hook their marks and then disappear into the ether; they do not want their postings to languish for for all to see.  It is not a deal-breaker, but should be considered along with other potential red flags.

3.  Be wary of "private" auctions which do not disclose the names of the bidders.  This warning is no longer valid because eBay themselves are now shielding some of this information.  Probably a good thing on their part.

4.  American laws do not apply anywhere but in America.  The bad guys know that.  So use great care, if you must proceed at all.  Especially do not deal with anyone who is "temporarily" out of the country. 

5.  If the sale is supposedly in the United States, make sure the bid amount is in American dollars.  Non-American currency values are printed in italics on Ebay, so they are easy to spot.  It doesn't make much sense for a guy in Texas to convert your bid to Euros, does it? 

6.  What is the return policy, and how much do they charge for shipping?  Don't be impressed by money-back guarentees and dirt-cheap shipping.  If it is a scam, they aren't going to give your money back and they aren't going to ship anything.

7.  Look at the seller's feedback.  Carefully!  Don't just look at an impressively large number and call it good.  There is a ton of useful information here if you take the time to really investegate. If it is a stolen account, they are somebody else's points anyway.  But you can learn a lot:

A.  Has he bought or sold anything recently, and what was it?  Accounts that have no recent activity are much more likely to be stolen accounts. 

B.  Is this auction consistent with items bought and sold in the past?  If a buyer of $5 trinkets is suddenly selling a $4000 drum set, take note.  It may be valid, but it is suspicious.

C.  Is the item location the same for this sale as for his last sales?  Does the location actually exist, or is it some fictional locale?

D.  Are his accepted payment methods the same as his last sales?  If he accepted PayPal before, why is he insisting on a wire transfer or cashier's check now?

E.  The seller's positive feedback points for purchased items are not very useful to you as a buyer, so don't over-value them.  

8.  What else is the seller posting for sale at the time?  Follow the link for Seller's Other Items.  If they have a large number of unrelated big-ticket items currently for sale (musical instruments, computers, cameras, bicycles...) that is very suspicious.

9.  Look at the photos.  Are they just stock pictures from the company web site or are they apparently authentic photos of the item?  Scammers can get pictures of anything from the web, including from other Ebay auctions.  If you have seen the exact same photo in other auctions, be suspicious.  Also, look at the backgrounds in the photos.  Does it look like they just took their digital camera into a store and started clicking away? 

10.  Talk to the seller!  How?  Place as low a bid as possible on the item.  Bidding gives you the right to view the seller's contact information.  Go to this link, and paste in the seller's name and the item number.  Ebay will e-mail his contact information to you.