I bought Vista from a US seller on ebay around a month after its release. I got it for some money (lol) and I've never had a problem with it. Activation and authentication has always been fine. However, the COA has a rip on the barcode which has always left me suspicious of it. It looks like it might be innocent, like a label was on it and got took off taking some of the COA with it but as I will be wanting to sell my copy I need to be sure it's ok.
Here's a close up of the COA - please let me know what you think.