I think they should also stay somewhat active. Not just a one time goal. But that could honestly be true for every section, not just TBR.
You could do some sort of payment system to be a member, but based on credits. Posts and replies that are "liked" earn credits. Report your own sale with domain & price and also earn credits. Just posting your domains for sale doesn't earn credits in fact it should probably cost credits.
Credits are deducted each month for your membership fee. If your balance runs low and you don't have enough credits to pay for the next month's membership, you can pay in $CAD to top up your account. And when you first sign up you can get a certain amount of complimentary credits to start with.
Members who participate are rewarded with free access ( which is fair since they're the one's generating the content). Lurkers have to pay $$$.