rogdcam: You are 100% correct. My state just implemented sales tax on eBay sales October 1st and I was pleased that when I purchased stamps from the official USPS store on eBay, there was no sales tax levied.
eBay seems to exempt the entire postage category, at least for my state (Iowa). If you buy discount postage from a non-USPS seller, for example, my eBay cart doesn't show sales tax. Other stamp categories are taxed, but postage isn't.