Some merchants think a seperate merchant forum or gallery is bad for their sales. But that depends on how it is handled.
Quite often I don't buy a product because I don't like one detail. In a seperate forum this vendor can post many making-of-pictures and I can tell him what I like and where I want some improvement. But maybe they don't want my input?

On second thought this forum isn't a good idea. If more people sell good products I would have to buy more. ;P