If separate forums within the Buying & Selling section is a harder approach, perhaps it could be addressed as part of the "RULES" to include one of the following IDs in the thread header:
FS: or FOR SALE:
WTB:
WTT:
PRODUCTION RUN:
So an example would be -
PRODUCTION RUN: The SOLDEN Jacket is back!! (ROYALE Filmwear)
That way, if a member wants to look for all the Production Runs, they could go to the Search feature, and just type in "Production Run" and select the "Buying & Selling" section. In theory, if every post/listing has that in the title, the thread should come up in the search, and BINGO!
Yes, it requires more mod activity to ensure that users adhere to this policy, but it would then eliminate the need for the sub-forum sections.