I just sold and bought wheels on this forum.
1. Research, research, research. Make sure you know exactly what you want (size, look, price...etc) before you pull the trigger. Once you know what you want, then shop.
2. If unsure about fit. Ask! ... or do more research! Many people on this forum have changed their wheels so hopefully you can use that as a type of reference to whether or not the wheels will work / fit on your car.
3. Use paypal. This way you are covered for your purchase!
4. Shipping cost can be anywhere from 200 - 500 dollars (when shipped with tires). You can use / have the buyer use ShipNex. Probably one of the better tire shippers around and they're way cheaper than other rates I've been given.
I mention the shipping costs because you can use it as a bargaining tool. Cheaper shipping = cheaper wheels!