Ten years ago when I sold comics on ebay I used the exact approach suggested by Broke as a Joke.
Exactly as he said, I gave a 3% discount for checks, and a few buyers took advantage of it (maybe ten per cent of buyers, particularly on larger amounts).
Either nobody turned me in or complained to ebay, or I was too small time for them to care (maybe 800 auction over 18 mos) but they didn't just swoop in and cancel all my auctions (as they had done in the past because I'd mentioned "esnipe" or something, I forget, I broke one of their rules and 20-40 auctions just disappeared into cyber-ether).
Re: the 3% cash discount, I happened to be a lawyer, but I thought I was just being clever, not lawyerly. ; )