My grandpa had a saying
No tickie no washie.
In other words you pay before services are rendered or goods shipped.
That being said , most of the folks on here are honest, however if i sell parts, or even give something away and only ask for shipping reimbursement, i ask for the money first. Some people will say thats crappy, but I have also had people want parts and i box them up, take the time to get a shipping price , and never see payment. So i take a page from my grandfather no payment , no parts. Simple as that.
Just my .02
Hope this helps you out
Matt