If it's a local deal, have someone go w/ you if you can. If not, make sure someone knows where you're going to be. If you're selling something locally you can choose to meet them at a public place if the item is small and easily moved.
Deals online are a little different. The main thing you have to remember is to just be honest and communicate. Pay people and ship people their items in a timely fashion. If problems arise, tell them and try to fix them asap. You can be stern, but always be polite.
You'll always want to describe your item honestly. Pictures are a huge bonus. People want to see what they're buying. Be ready to negotiate who is going to be paying shipping charges and paypal fees. I generally price all my items shipped and I eat the paypal fees. If someone haggles w/ me I start negotiating those items from there.
Some ways to protect yourself. I generally accept paypal because it's easy and most people have it. I only accept personal checks from people I know. If a deal is w/ a stranger I accept postal money orders. When buying from someone and paying w/ a money order always use a postal money order and send it certified. That protects both parties.
I've run into a couple of problems over the last 10 years buying and selling car parts online. I've always been able to resolve it. Sometimes it's cost me a couple of dollars, but I always try and uphold my reputation as being honest and someone people want to do business with.
Good luck,
Steve.
