Are you sure? One would expect that most new product launches or major sales would be linked to by users for free. Additionally, all the major vendors are already linked to in the sidebar wiki posts...

Major vendors, yes. But there are a lot of small run or custom accessories that would sell quite well if they had more exposure. The wiki in the sidebar is good, but what about when a company comes out with a new set of keycaps? Getting that information out can often be iffy, but ads in a major community would make it easy.

Post to /r/MechanicalKeyboards with "I run a Mechanical Keyboard company, AMA", and you'll get an epic thread. Post and ad and it might get 2 comments.