Do your homework buying and selling. Please make sure the people you buy or sell to are properly vetted. There are a lot of people that will take your money if they have a chance. Feedback forum on qth, qrz, and eham have various feedback available. Another way is to google their callsign. The disclaimers can bite you as well. An example is to sell a rig using Paypal, get an email saying it has a problem, Paypal refunds the money and get a box of rocks with no recourse. I could go on and on.
Good trading buying and selling,
Terry N8ERM