If you are selling ( not scamming ) ask for money first. Do not give them the items first. They are gonna run off with the items than pay you. If you are buying item first and then money. It just depends on the kind of seller and the kind of buying to get scammed.
Scamming is allowed. The point of the server is greifing, looting, raiding, and scamming. Just be careful if you don't want to lose your stuff.