I don't see this as a scam, just a potential problem buyer. For starters, the fact that you're not in KY is irrelevant, because sales tax is determined by where the item is shipped to, not where the seller is. Second, eBay determines whether an item is taxable based on three things: shipping destination, laws of that state, AND the category it was listed in. If a non-taxable item is listed in a taxable category, eBay charges the tax. eBay also sometimes makes errors in their tax setup, and sometimes doesn't keep up with changes in the laws.
Whatever your buyer's problem, it's with eBay, not you. If they buy from you, they may harass you for a sales tax refund, even though you don't collect the tax. If this happened to me, I'd block them. I might also send one last email explaining that eBay is subject to KY's marketplace facilitator law, and THEY are the ones collecting the tax. Suggest that the buyer ask eBay why THEY are taxing him on non-taxable items.
