Most used gear is sold "as is", and the buyer is the one taking most of the risk. If there was no obvious issue with the lens when you sold it, and now the buyer is claiming it needs to be 'fixed', it does sound like a bone of contention. If you are sure the gear was properly working when sold, and stated so in the sales posting, AND there were no issues with shipping, then the buyer gets what they get. If they feel that something was not working correctly, have an estimate provided from Canon (or whomever) for the work to be done and go from there. If you are unsure of the lens condition when sold, you may want to offer to pay the repair, or maybe split the repair costs with the buyer if you feel your lens really did not have any issues, but you want to be reasonable.
Even on the GetDPI B&S section, Jack has posted the following:
*** GetDPI.com offers this as a free service for our members and makes no warranties or implications regarding the veracity of the posters or items listed for sale, as such USE OF THIS FORUM IS AT YOUR OWN RISK.
LJ