So if you buy anything from me, you're going to pay me sales tax, right? I didn't think so. From what I know of the SCOTUS ruling on internet sales tax, the issue was whether the business had a physical location and whether or not sales tax could be applied in that transaction.
I'm not going to go look it up, but I don't believe it had anything to do with private party purchases. If it did, obviously, I am wrong. If it did not, you would think that GB could easily determine who is a vendor and who is a private party.