It's smart to shop around locally so you can inspect the lens in good light before purchasing. Make sure there are no scratches or stains on the front or back of the lens, and take into account if the seller seems to know what they're doing when they're protecting the lens with the lens covers.
I don't know if I'd trust just any retail store with used camera lenses, some of them may have no idea what they're doing.
Ebay may be a good spot to start if you can find someone with a good reputation of selling camera lenses. Asking them questions and looking for a careful description can go a long way! And of course you have eBay buyer protection if someone fails to disclose that there's a scratch or something.
(Buying one that has been protected with a filter or a lens hood may be a good idea!)