Acaeum is pretty much it for the collecting community.
If any particular facet finds itself with an overwhelming body of knowledge and an inclination among collectors to develop the forum further, then subwebs are developed by those with that knowledge base and the time to dedicate to its development.
That is how the Judges Guild and Planescape subwebs came to be.
There is a lot of 3.xE expertise on Acaeum, not least among the many resellers who know their stock and their buyer. At the moment the third party 3.xE discussions tend to occur on the non-TSR and Appraisal sections, whereas
WotC matters crop up on the TSR section.
I have noticed most discussions tend to revolve around third party publishers and their offerings, what becomes collectible fast, and what does not, and very little discussion is to be had about
WotC's core products. This may be because, for the most part, 3.xE is more of a speculative investment on the part of resellers, than a core collectibles goal on the part of fans.
This week I've been mostly eating . . . The white ones with the little red flecks in them.