I prefer a store that stands behind their gear. Keh has always been good to me. eBay is alright, but let the buyer beware. Many sellers don't even have the expertise to guarantee an item. You have to read eBay descriptions like a lawyer. If it says "worked last time I used it, but that was a year ago.." or something like that, watch out. Also beware AS-IS and no-returns auctions.
I've had some good experiences, but also some bad ones. Here's a list of a few bad ones: 1. Nikon 16mm F2.8 AF from a studio photographer in Cali. Wouldn't focus to infinity, which makes it pretty useless. He said it had just come back from Nikon Factory Service. He did accept a return, but gave me a real hard time about it, telling me I didn't know how to use it. 2. Tokina 70-200mm f2.8 MF with a broken bayonet mount. The seller refused a refund, so I had to send it to Tokina for repair. 3. A Nikon FM2 that was not in EX condition as stated. It had many wear marks. I kept it, and had to get the film transport worked on only months later, because it had ben used so hard. 4. An ipod that was supposed to be in EX condition. When I got it, it looked like it had been through a war. Twice. Seller was very difficult and I eventually had to threaten a chargeback on Paypal before he would accept a return.
I like eBay much more as a seller than a buyer. If we could get rid of the bad sellers, it could be really great, but that will never happen, so Let the Buyer Beware.