I have sold a Sigma 18-50 f2.8-4.5 OS HSM to an internet buyer. The lens worked fine, but after receiving it today, he said that whenever he turns on autofocus on the lens the camera has a glitch in the autofocus area, switching very fast between AF and M in the menu.
I have never encountered an issue like this with the lens. I believe it could be the AF / M switch broken or something electrical doesn't work on it.