So, I just sold a Sigma 24-70mm f/2.8 on eBay, and I was honest in my description. I said that I'd tested it out on both an XSi and a 5d2, and it focused pretty accurately on both, but at 60-70mm in low light, it was slightly hit-and-miss. In my opinion, that's to be expected. However, my Sigmalux performs differently on my 5d2 (+6 MA required) than it did on my 40D (perfect), so I worry that the lens I sold may have issues on the buyer's camera even though it was OK on mine. So now I'm playing the waiting game and crossing my fingers that everything's going to turn out well!
So, just out of curiosity, I want to see what you all think. Given an honest description of a lens as focusing well, is front/back-focusing a valid reason for returning a used Sigma to the seller?