Here are the rules that have kept me out of that category so far:
- Words like mint, good as new, only used one hour, etc don't mean much. Photos of gear, with the ham's call sign visible on a QSL card in the photo are much better indicators of condition.
Know how much the gear is worth. If the offer is twenty or thirty percent below or above market, it is probably a scam.
Is the buyer or seller who they say they are? Always get a physical location, phone number, call sign, and email address. Then check to see that they are all real and match up. As a backup, check the IP address of the email your received, which is hard to forge.
Do not buy gear from a seller that wants a cashier's check, Western Union, Paypal "friends and family" or other non-recourse method of payment.
Do not deliver gear to someone that has paid with a money order of any sort until you cash it. They are easy to alter.
Whether you are buying or selling when you make a deal, be very very clear about what is included, item by item. If the gear has a serial number, make sure it is part of the list. Include shipping instructions: carrier, type of shipping, insurance.
If you are buying, check the gear against the list as soon as it arrives. If you need to check that the gear is working, go through every single function, mode, and menu. If it is expensive, have a reputable shop check it out for you.
Last of all, for expensive equipment, make a phone call to the seller or buyer early in the process. This will eliminate one hundred percent of the long distance scammers and almost all the folks that can't deliver what they promise.