1.Check a seller's feedback rating before bidding. Do not bid if sellers has low/no feedback or alot of negative feedback.

2.Ask the seller a questionto get an idea of what type of communication you can expect from the seller. Do not bid until all of your questions have been answered.

3.Know what you are bidding on before you bid. Chech the fine print. There are sellers that use clever wording to rip people off.

4.Check the terms. If you do not agree with the terms, don't bid on the item.

5.Make sure you know what the shipping will be. Do not become a victim of overpriced S&H.

6. Obtain The Seller's Contact Information:
After bidding make sure to copy down all of the seller's contact information. This can be found at http://cgi3.ebay.com/aw-cgi/eBayISAPI.dll?MemberSearchShow
If the user is unregistered or blocked from ebay, before or during the transaction you will be unable to obtain his contact information. So it is better to have it as early in the transaction as possible. Also try to get a way (other than email) to contact your seller.

7.After winning verify the seller's contact information before sending payment. Make sure you keep an eye out for any inaccuracies. If a seller's contact information is at all fraudulent, you have no further obligations to complete the transaction. The majority of scam artists use phony contact information. Discrepencies in contact information should be reported to http://pages.ebay.com/help/basics/select-RS.html

8.NEVER USE WESTERN UNION OR SIMILAR SERVICES TO PAY FOR AUCTIONS.
These services are meant to be used between two parties who know each other personally. Western Union is not safe for auction payments, and I would estimate at least 9 out of 10 sellers who only accept Western Union for payment are commiting fraud. 

9.Know Your Rights As A Buyer. US and Canadian Federal Law prohibits the use of deceptive or misleading acts in commerce. Sellers are required to advertise your product or service and the terms of the sale honestly and accurately. If a seller fails to do so he/she is breaking the law. Sellers are required to ship the merchandise within the time frame designated during the auction or, if no time frame is specified, within 30 days. If a seller can't meet the shipping commitment, he/she must give the buyer an opportunity to cancel the order for a full refund or agree to the new shipping date. Sellers are completely responsible for an items safe delivery whether or not insurance is purchased.

10.You should never leave feedback until the transaction is completely finished. This means either both parties are satisfied or one (or both) parties refuses to participate any further in the transaction.
Often if you leave negative feedback too early the other party may become reluctant to assist you any further in the matter.