"their fees should be based on sales price "
It is, the total sales price. What the buyer actually pays for the item and that includes taxes & shipping costs. It's the same as if you bought it from a store. For example: The retail price of the generator on the shelf has the shipping to the store, taxes paid, etc. built into the retail price on the sticker and (if you are a sales tax state) more tax. eBay is just a big store that shares it's info because it's participants are both buyers & sellers on the same venue.
BTW: The buyer does pay 13.25% of the added sales tax.