First off, where are you planning to trade and how? If it is via Ebay and Paypal, then there a certain amount of precautions one can take to mitigate fraud.
If you are planning on selling via Dakka or a similar web trading site, then it is Caveat Emptor, I'm afraid. There
are some steps you can take that will help you; have a read through the
Swap Shop Rules and Guidelines (a couple of posts down, Ketara has set out some handy trading tips.