I have one, Sigma 18-50 f2.8 EX. Its the second one I have had as the first was not very good. I took it back to Jacobs and we did a test on it. The first was not good at f2.8 but the other was much better. hey let me swap so I ended up with a “good” one. They said they couldn’t really tell but the difference was clear to me.
Probably best to buy from a shop then me thinks rather than on line.
They did admit later that they had had similar problems with Sigma lenses. Basically poor quality control.
Focusing is efficient (on my one) and its fine throughout the apperture range but quality drops off at f2.8.
Yours for 200 delivered. mint. boxed (probaly be able to find it), and with the bag thing for it.