Sellers: Please be as clear as you can. State a clear price and whether postage is included or not. State whether you're happy to ship abroad. Don't cover problems and most important, you MUST be honest with people.
Buyers: Be sure you can afford it. If the seller is unknown - ask mods. Take your time when sending address details and make sure that they are correct. It's SO easy to make a mistake.
Both: Don't be slow in sending items or reporting that items have arrived. It avoids a lot of anxiety. Please report on the transaction accurately and fairly for other users to use as a reference for later transactions.
Feel free to ask questions in order to clarify what you may be buying.