The OP's point about eBay being unethical in the way they charge their fees is understandable but I look at it a little differently. 1) The fee structure is transparent; it is in their user agreement, 2) As pointed out above it circumvents any games a seller might play with pricing and shipping, 3) If eBay didn't charge a percentage on shipping and sales tax they would just raise their fee on the price of the item. Instead of 13.25% perhaps they would charge 15%.
I've sold quite a bit of stuff on eBay over the years and even though I'm irritated at some of their policies, I continue to use them. One of the most important benefits they provide is that they have negotiated very attractive shipping rates. If you book you own shipping you will see that it is much more expensive than going through eBay.
You may have a complaint about charging fees on shipping and taxes but IMO the fee that eBay charges compared to the service they provide is still a bargain. I'm old enough to remember the days before eBay when selling a piece of stereo gear was a big hassle. You took your component to your local stereo store (if you had one) and stood there powerless while they offered you 20% of its value if they would take it at all. Your only options were to put it in the closet or take it to Goodwill. So you took a deep breath and mumbled "OK." Paying around 15% or so to achieve an honest auction value for an item is a much better alternative in my experience.