[Rob]

What version of .NET is the server running on? If you host a WCF service inside the server, you can easily cache results with just an attribute. You won't be exposing the server to any less lag potential than the server itself does.
I'd make the page only accessible if you've authenticated into the forums.

I mean, really... over a hundred people are sending at least 10 requests per second to the server... 1 ajax request per two seconds won't do shit to performance if you're careful.

Right. Authentication is also a good requirement. Not familiar with WCF but does that just add a Cache header to the http response? By doing that the server is telling the browser that this page is valid for up to x minutes or whatever and the browser will honor that time and not re-request the page until that time expires.

With caching there will be almost no impact on the server. If you send cacheable gzipped json with client side rendering we're talking probably 5k of data (estimate of all items on all vendors) to a user every x minutes, x being the cache time. By looking at Razor stats, the server sends every client that amount every few secs.

Done right this would have minimal impact on the server.