The only way, and i do mean only way, that using eBay to buy software would be of any benefit is if you know, and have proof that the seller is giving you an unopened boxed version of the software. Give a copy of the receipt and when you get it go over the package with a fine tooth comb. Make sure that the package is the same as it looks from a retail box at a store to make sure that they didnt open it and re-wrap it.
I dont trust even established sellers. The fact is this, that system is just like the rep system on this site. It can be used and abused. They can have 5,000 positive feedbacks. But how do you know that they didnt create junk accounts or have their friends "buy" items from them and give them positive feedback? You dont.
The simple fact is why take the risk. Memory was lucky that he got his copy exchanged. But there is nothing saying that the person didnt turn around and sell that same serial to someone else. There is no way to know for sure. Retail can be activated on up to 3 machines before it throws a fit. So a seller could sell the same serial to 3 people and have each person activate it with no problems. that doesnt mean that when 1 of them tries to transfer it to a new machine, which is legal within the EULA, that it will activate again.
If you have to dispute it, there is no guarantee that you will get your money back. Microsoft isnt going to be of any help cause they dont consider eBay a legitimate source for selling their software. They will try to sell you another copy or tell you to buy another copy.
In the end you could easily be paying more than what it would have cost you up front. If you wish to take that risk, by all means do it. But i would rather be safe than sorry.