I disagree with the 250 post count, I post on average maybe once every 2 days. For 250 post count, that's going to be over an entire year before getting access to the marketplace.
I think 100 post count is more than enough.
Other limitations would be cool I guess. However, post count I think should remain where it is. Forum activity doesn't even really translate into trusting people much either. If you're a chatbox user, I'm sure that the people you trust the most are those that are active in the chatbox.
As you can actually get a feel for them as a person and develop a friendship; that's much more difficult if not impossible on the forums.
Personally, I don't think post count translates into any sort of trust at all - I also don't think it will prevent any scams. Anyone dedicated enough to do 100 posts, will happily do 250. But now they'll scam more as they've put more effort in.
I think that raising the limit to 250 will slow down market activity a lot.
Otherwise, it's a nice idea to have an application system into the market. I don't think it's realistic though as there's not many staff members and having to deal with the current work load with an additional application process for members will be too much work. Which would have negative impacts on other things that need to be handled.