Surely they are rated on Ebay and that would give you some idea?
Personally although I buy quite a lot through Ebay I hesitate to buy things like a complete PC. Its partly the returning the item that puts me off I must admit but also wondering if the company/person will be there in the future if something does go wrong.
Because of this I prefer to buy from a company that I know has been about a long time such as Novatech. I know theres no guarantee that they will still be there in 6 months time as businesses fold all the time but theres a much better chance they will still be there.