[Nonfractal]

At least on ebay everyone has feedback ratings, I avoid anyone with negative or near zero feedback.

Ebay +PayPal and a good seller history. Minimum.
Never accept that a seller can blame the courier for your goods not turning up. 
It is a seller's responsibility to get your goods to you.

Only once has a seller tried to rip me off. 
It took time,  but paypal forced the refund even though the seller closed their eBay and paypal accounts.
AlanW