just make a standard "No Bumping Sales threads within "X" amount of days" rule.
Seems to work out fine everywhere else on the internet.
True, otherwise someone agressive can hi-jack first page and it's "according rules".
I'm also OK with a single sales WTS thread/single WTB buyers thread allowed.
Than someone else will ask for one more tread-per-person - WTT. Plus above opinion to have thread for Famicom and one thread for other stuff. So it's already 6 threads per person.
IMO to limit just 2 threads per person it should be WTS:thread and WTB/WTT:thread. "Subject" is long enough to describe "what are you looking for" proper, not just "want to buy some games".