I've bought a teleconverter from lensrentals.com. Was exactly as described, if anything their ratings are conservative. Most of the lenses have a 90 day warranty, which is actually better than most places selling second hand gear, as well as the 3 day inspection where if not happy return for full refund. Just check the details in the sale listing.
As far as I am concerned, if I was buying a second hand lense, it would be one of the few places I would actually consider.... they can't afford to rip people off, as they live (and die) by their reputation.
The owner actually a forum member here, but cant remember his user name at the moment.
7DMkII | 7D | 450D | Canon 18-55 IS | Canon 55-250 | Canon 100-400L MkII | Canon 100-400L | Canon 24-105L | Canon 50mm 1.8 | Canon 1.4x TC II | Tokina 11-16