For me, the rules are the same, no matter what product I buy on eBay:
Buy from a reputable Seller with a high Positive feedback rating.
More importantly: Check out any Negative feedback for possible patterns & exactly what the problems & recurring problems have been. You don't want to be the NEXT person to have the same problem, especially if it's a pattern.
Read the product description CAREFULLY and note whether the product is New or in "Like New" condition, or an item that is "Like" the name brand product you actually want to buy - but NOT the real thing. And what is the packaging condition of the new product, could it have been possibly opened, tried out, used (messed with,) then repackaged & now sold because the Seller didn't like it.
If it passes all these criteria, who cares where it was bought?
Most reputable Sellers with excellent feedback, will not purposely risk a negative rating by selling something different than what they have described. And as a longtime Seller & Buyer myself, I know there can be a number of reasons why & how someone ends up with a brand new item that they decide to eBay off. I care more that it is the real thing & in brand new condition.