I voted low, because I don't think it's entirely about post count- 100 worthless posts in the BYPC thread doesn't mean anything...

The idea of blocking until someone has 3 positive feedbacks seems like a good idea, but what if they're in a bad spot, and need to sell something?
No, they're not 'entitled' to hock their wares here, but it's assuming they're not worth anything because they're down on their luck- not cool.

it would be great if there was some way to earn it- like a "buy a MOD a beer" bonus (or other face-to-face, like at a shoot, or some get-together?)

Many new guys will flake because they're uncomfortable, and going into new territory by coming here- fear of the unknown. Think back to the 1st gun you sold to someone you didn't know... did you ever wonder if the next time you saw it was going to be with "People's exhibit A" tag on it?

just something to think about...

The minimum post count is a way to reveal something about them- I've been on other boards where they post "9 more posts to go" etc... so it's very clear their intentions. That's the other side of minimum post counts, and we've seen it here already. Make it apparent in the rules that this won't fly...