I've had no problems with Dabs or Amazon customer service-wise, both have great service and have been happy to help with any enquiry i've made. Same goes for GAME as well. E-buyer has good service, well they have for me anyway.
Some of the worst i've had was from E-bay, which surprised me. Somebody said i sold dodgy stuff and e-bay threatened me, and were not in any-way going to listen to my case. I did go to shut my account down, but it got refused until i payed something like £150 back to the guy who put claims down for dodgy stuff, even though the item page stated "no refunds" and payed e-bay for something.