If I am to understand you are attempting to sell SOME items TAX FREE, and others as taxable.
I think our accountant would call this a case of ONE PAIR OF PANTS- WITH TWO POCKETS. And, as long as you are wearing THOSE PANTS, you'd better be charging tax on EVERYTHING to stay away from problems.
Most of the time it is easier to open ANOTHER eBay account to stay away from the conflicting issues this situation poses. Are you a LTD company? Is your eBay account registered as such?
If so, and you are, are contemplating, running NO TAX sales through that account, I'd be quickly seeking advice from someone with much more knowledge. Looks like a recipe for disaster.