I agree taxes can be a total pain. If the publishers didn't specify any city, county, etc extra sales taxes then I'd think that their presence isn't in a city/county that has any. Many don't charge an additional sales tax above what the state charges. If the publishers presence isn't in one of those localities that charges these additional taxes then they don't apply.
On another note. I was helping a friend get some books, published by Samhain, this weekend. I went to your site thinking I could finally buy something their as I'd remembered seeing some of their stuff when browsing around. Unfortunately you're only carrying their books as PDF's.
I know for a fact they publish ePub's of their stuff. You also might have a hard time with some of those smaller pubs if you're only going to offer one format since many of them offer multiple formats directly and also through Fictionwise so you can go back and download other formats later if desired.