Big assumption there.
I buy my used lenses from Fred Miranda. Haven't had a problem. It's a community of photographers who just want to buy or sell the lenses they don't use or need anymore.
Bought my 24L II and my 10-22mm there and I've been pleased.
I'd have thought somewhere like lensrentals would be OK to buy from as their kit will be serviced everytime it goes out for and comes in from rental.
i buy used lenses almost exclusively, but i don't buy them from a rental house.