When you hit the buy it now button, you knew that he was going to charge you £6.75 postage so by BIN'ing it, you agreed to pay his charges. I do see your point, but its not as if he hid the charge from you.
Generally people don't know what it costs to post stuff, so they guess. He guessed at £6.75, and you came along, quite happy to agree to that cost, and hit the BIN button.
It always pays to query high shipping charges before buying something on e-bay, but then if the price is so good that you don't want to miss out by procrastinating then surely in your mind, you've already justified the £24 for the shoes anyway?
Leaving you poor feedback is harsh though. With the new e-bay system, you can give him a bad rating for his shipping costs, which will affect the way that future stuff he sells is listed.