One thing people would potentially expect for their money would be some sort of regulation in the selling area. If that were to happen, I believe you can say bye bye to the selling forum which would be a shame.
Yeah, no, charging for membership is a ludicrous idea. It would...rightfully...give people a sense of entitlement and ownership that they absolutely should not have.
Charging for strikes, on the other hand....
Enforceable, how? Fining on the honour system? I guess, if you don't pay, you have to create a new identit- heeeyy, wait a minute...
The simple "solution" is to require a non-free email address or phone number or real name or something like that, rather than the "it takes a minute to sign up and i probably won't even need a real email address" system the board has now. Remember that the boards are kept as a convenience, and there's exactly zero interest by tPTB in putting any man-hours beyond what it necessary to keep the boards technologically functioinal