UsedPhotoPro (Used department of Robertscamera) is offering a 10% coupon for all their used camera gear. I'm not sure how long it lasts or any exclusions, but I tested it on a few pieces of Pentax gear (50mm f1.2, 35mm f2) and it works. They have a lot of lenses and a few digital bodies. Also good for other brands, obviously.
I have purchased two items from them in the last few months, an Xpro-2 and a Pentax *ist DL. Both were a great price and worked as expected.
Fall Expo Savings
Coupon code: EXPOSAVINGS