Yes, I have used them extensively for buying and selling Nikon, Sony and Fuji. They are based in the UK, where they are well known, but opened up in the US a few years back.
They tend to pay about 5-10% more than others when buying and their used prices are similarly on the low side.
Their gear quality is accurately described and usually they have actual pictures of the actual item you are buying (i.e. B&H and KEH generally do not so you base your purchase decision on rating system.
The only downside/upside is that they are very selective in what they sell. While some used dealers will sell items that are worn and abused, mpb generally will only buy or sell things that are in 9 or greater condition. If you are a buyer, this is great as the items are always in good condition unless pictures indicate otherwise. If you are a seller, they only buy lower shutter count and lenses with few or no signs of use.
All of the lenses I have bought from them were excellent in their original boxes. The only signs of use I found of them was a minor scuff mark on the lens hood.
For Sony lenses, they tend to sell out what they have for the higher end lenses very quickly. I have been looking for a specific lens and they turned over the one I was looking at within 10 minutes of it being posted.
