I bought a used copy of the Sigma 18-50, it arrived -unusually- with a filter in place (I remember this from my photo retail days, staff got to keep the decent quality used filters off of lenses we bought in, so we could get the mark up on the crappy filters we sold) I had to use the hacksaw method to get the filter free (I should have in actual fact rejected the lens, as time would tell)
About 8 months in the well performing but sloppy feeling lens broke. Not dropped, no impact, just the zoom barrel slid out of it's helix and would not pop back in. Right in the middle of a video shoot with no back up.
My used bargain lens required an £80 repair, to be fair it arrived with a new barrel and was exceptionally well calibrated (focus faster, sharper corners)
I know from experience that a dropped lens can appear fine but have a problem unseen that will manifest itself at the worst possible moment.