Outa the 200 or so deals Ive done Ive been ripped off 2 or 3 times. But only on small cheap items.
If its expensive then use a credit card as most have insurance policies against internet fraud. I always do this for guitars as they are expensive.
Also look at the sellers rating! What have other buyers said about him/her?
Ask questions!! Email the seller and ask everything you need to no! Some sellers wont mention things that might put you off so if theres an area your unsure of then get it cleared up!
Basically the main thing is to use a credit card when buying, because then you have insurance.