tough question. if you don't have a reputable store to buy from who will honour basic human decency if they sell you the wrong/faulty part, then you simply have to try common sense approach at the time of payment.
as i've handled a lot of retail packaging i'll talk about that specifically. in australia even though we have excellent consumer protection laws, people STILL look at the box of components with suspicion before they part with money. does the packaging look brand new? or has it been re-shrink wrapped? is there a lot of wear and tear on the box? is it very dusty or dirty? does the salesperson seem genuine, or is something "not quite right". remember until you hand over money, you can always walk away!