I've crunched all the numbers just like you have here. As a print seller on Ebay, I understand the final percentage that they take, and agree that it's too much.
I think Ebay would be much better if they took a higher final value fee from people, rather than taking money just to list something.
That is really where Ebay is making all their money, from stupid people that make high priced auctions that will never sell.
I used to sell my prints for 50-75 dollars on Ebay, now I sell them for 175 and will soon be raising that to 250. My prints are selling just as well at the higher price as they were when they were lower.
I think all the other self representing photographers on Ebay need to raise their 40-75 dollar print price, as it's only hurting the photography market. There is no reason to be selling work for that cheep.