Add to the other three Henry's of Canada, and Cameta
Also check out the marketplace here. The Photographer's Marketplace - PentaxForums.com
Generally speaking, the deals I've seen here are pretty darn good.
KEH has the largest selection, and they are very good at returns. I have bought many lenses from them. Also check out their "In Transit" section, sometimes you can get a early shot at some pretty popular lenses that way.
Henry's (in Canada but they regularly ship to the US) has a pretty good selection and are also good at returns.
Adorama is my primary brick and mortar store as I live in NYC, I've bought about 5 or so used lenses from them and haven't yet had a bad one.
I haven't bought a used lens from Cameta, but I know ppl who have and they've been satisfied with them.
B&H is also in NYC, but I don't patronize them because I find their sales staff to be pushy and sometimes rude. Many, many here tho have had good online experiences with them.
If you are looking for lenses, don't forget to check out film bodies. Often there will be a good used lens attached to a less than appealing body, and you can get both for less than you might pay for the lens alone.
I have bought and/or sold over 20 or so lenses, most thru KEH and Adorama. I sell thru Adorama primarily because the ppl there know me and give me a very good price if I use it to buy more camera gear from them. But in all the lenses I've bought I've only had 3 bad ones and have never had a problem returning them. But then again I generally stay away from eek bay.
NaCl(I may be poor but I've got a lot of glass)H2O