Hi Shiftdelete,
Welcome to the Mattress Forum! 
The membership list here includes the retailers or manufacturers that are invited members of the site and the discount or bonus that each of them provide to the forum members here.
Just in case you haven’t read it yet … I would start with the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you make the best possible choices … and know how and why to avoid the worst ones.
There is also more in post #13 here about the most important parts of the “value” of a mattress purchase that can help you make more meaningful comparisons between mattresses.
Phoenix