Currently, on the first page of the marketplace, one seller has donated to MR. This means that if you required a donation, it's likely that many people would go elsewhere. So requiring a donation would kill the marketplace.
Post count doesn't mean much. Someone can post all day on a forum, and still be a scumbag. Be cautious when purchasing, check reefer ratings, and leave reefer ratings for people.
There's nothing wrong with leaving a good or neutral rating with the comment "showed up 30 minutes late, but decent guy". If people start doing this, you'll be able to determine who you want to deal with. If I see that 5 people said that the same guy is always late, I'm not going to deal with him if I'm on a tight schedule, but maybe I wouldn't mind on a lazy Sunday. It comes down to everyone who buys and sells to make the community better.
We're all adults here, and we shouldn't need mommy and daddy to tell us when we can and can't do things.
On another note, if you think MR is only after money, then don't pay to be a vendor. No one is forcing you to pay a dime as a vendor or a member so stop whining about things. If your MR vendor fees are so high that you're not making money, either stop paying them, or take a look at your business model, and realize that maybe you just need to run a better business. Advertising isn't cheap.