I've been dealing with KEH for about 10 years, both buying and selling.
It's been mostly good experiences, but I did have a nightmare once where they charged my credit card but the equipment didn't arrive. When I called to question them they said they had inadvertently sold the equipment to another buyer. They shipped a replacement for me and it was the wrong item. I returned it and finally got the correct item, but it took about two months to clear almost everything up, as I never did get a credit for the two extra shipping charges (The shipment they didn't send and the return). I was too happy to finally have the lens to worry about it.
Despite all that, I continued doing business with them and haven't had a problem since.