Top 10 list
1) Never buy stamps in a private auction or if the seller keeps his/her feedback private. Private auctions can be used to conceal shill bidding that artificially inflates the final sale price. Private feedback can be used to hide unethical dealings. (Note: Some new sellers use private auctions, thinking they do their customers a service. In those cases, a short note about the pitfalls of private auctions is enough to convince them to go public. And they are usually greatfull to receive that advice.)
2) Check the feedback of the seller. For stamps, anything
99% is a bad sign. Make sure to read all negative comments. They tell
about what service you will receive if something goes wrong. Also,
the feedback of those leaving negative and neutral comments--sometimes
not the seller's fault.
3) Look for signs of shill bidding. If some of the users you bid against have very low feedback, and if they only bid on the same seller's items, and if there is a great variety of items they bid on from the same seller, retract your bids, and report the shill bidding to eBay.
4)Be sure to ask questions about the items before you bid, if you
have questions. If the scan is unclear, ask for a better scan,
if the stamp being offered has a high catalog value or is commonly
reperfed, or otherwise altered.
5) Check the stamp image out very carefully for signs of damage, alteration, reperfing, erased cancels, etc. Has the seller mentioned any visible faults in the description? If you're not sure of the stamp, ask someone with knowledge of the area to look at the scan.
6) Read carefully shipping and payment terms. If shipping costs are unreasonable high, you may report the seller for "fee avoidance". If the seller does not clearly list shipping and payment terms, ask via email before bidding.
7) Are return privileges acceptable? Is the length of time permitted long enough? Will the seller give an additional extension if you want to have the stamp certified? An honest seller will give an extension to have a stamp certified, and will pay the cost of the certificate if the stamp is not as advertised.
8) Never bid on an "as is" auction. Not a used car, and not a stamp. If the seller can not guarantee the stamp, you don't want to be the customer. As a famous stamp alterer said it: "You're buying a pig in a poke"
9) Learn about the seller. Good feedback is no guarantee for
honest seller. Some buyers were willing victims, knowing that they
junk, and others might not know thay were defrauded. Use internet
engines, and ask on message boards if you are unsure about a seller.
10) Leave honest feedback. Many buyers always leave positive feedback, even when they know they were scammed. Doing so is a disservice to the community , and undermines the credibility of the feedback system.