Warehouse Express: I had a problem with a Leica filter and WE were very helpful, exchanging it for a Nikon one at their suggestion. Very helpful.
Norfolk Camera: I bought a used lens. It was as described. Friendly people too.
Digital First: I used to live nearby so I would pick up equipment in person. Very friendly and they often give a free extended warranty.
Fffordes: Generally good second hand gear, though it's best to ring them and ask them to describe the item over the phone so you know what it's really like.
Mifsuds: Good for selling used gear on commission. But I regularly had to push them to get the money once the item had sold as they never chased me up.
B&H Photovideo of NY: Worth it just to hear the NY Jewish accents and known by some as "Beards and hats". I've found them very good to deal with.
7dayshop: I used to buy film (remember that?) and batteries from them and they were excellent. I've heard bad stories though ...
Jessops: There are better (and cheaper) places ...
Grays of Westminster: Doubtless they know their onions, but too posh for an oik like me.
Maybe for the Queen and Michael Winner.
Assorted professional photographers selling in small ads etc: I've had numerous unpleasant experiences dealing with pros. The gear was usually thrashed, the descriptions were often blatantly dishonest, and the prices were too high.
Leif