topic where he can restrict newbies as per his wish.
This is the slippery slope I meant.
Newbies already have a similar limitation, they cannot post on the Serious Discussion boards. I am all for this.
I'd choose adding a new board with certain restrictions over restricting existing boards.
OP could then go into his "Post Filtering" settings and under "Marketplace -> Lending" could enter an expression like: exclude($memberRank < 2) (where 0 == "Brand New", 1 == "Newbie", 2 == "Jr. Member", etc).
I've seen similar suggestions before. Why not make use of the self-moderation feature? It could even be maintained by a user bot.