I'm guessing enforcement will be simply telling the user that they can't post with less than X posts, then notifying mods. Just as if he didn't follow any of the other rules in listing items. This is just one more thread that gets notified to them - I don't see it as being a massive strain on the infrastructure.
I like your post count idea,but one could post a hundred posts like in that strange post count thread(ask Porcupine about that one) in a day if they wanted.Maybe your three month waiting period might be better?
Never 100 in a day....
I agree with oakie muskoki on that.A waiting period is a good idea.If it is in the case as people have pointed out before,of a person coming into a collection (or many other scenarios ) and wanting to sell,then that time period could be put to very good use asking questions and learning here.
I can't think of a better place for a comic education with all the nice knowledgeable folks around.
If they cannot wait then pooh hoo and they can take the 20 to 30 % that most shops and dealers will offer them.
Oh,and thanks for the shout out.