Just an idea:
Why not have a dedicated sub-forum for each sponsor, and that is where all their ads go? So instead of having the current sub-forums of "Wheels and Tires", "Suspension and Brakes" for example, you have "JLeviSW", "SPP", "HP Autowerks", etc.
Or you can also take a hybrid approach. Leave the current sub-forums, but enforce a 72 hour bump rule, and also have the "Industry" sub-forums for each sponsor. You have to keep an eye for "shill bumping" too (having your buddy ask a bogus question, etc.)
Personally, I'd love a way to find out what all the products available from one vendor are, and the current system makes it tough.
Here is an example from a firearms website, the most active and popular one on the internet. They've got the usual "Equipment Exchange" as well as the "Industry" page.
"Industry Forum:"
http://www.ar15.com/forums/board.html?b=2
"Equipment Exchange:"
http://www.ar15.com/forums/board.html?b=7
Their selling rules:
http://www.ar15.com/content/page.html?id=301
Now you may be asking why look at a firearms enthusiasts page for idea, consider the site statistics:
E90 Post: Threads: 152,703, Posts: 3,499,928, Members: 45,815
AR15.com: 177,644 registered members, 132,882 are active accounts. 1,760,149 topics created and 22,237,817 replies.
Very busy place.....