I agree with the middle man principle, it will give the chance to build up a positive trade rating for n00bs.
It is the same conundrum with work experience, you can't get a job unless you have work experience, but you can't get work experience without a job...
This will give the opportunity for new members to get age verified, buy a gun, and not be turned down, ignored, or taking advantage of in the form of fraud, while on the other hand, protecting the seller from shaky deals.
As per shipping costs, the buyer sends the money to the TRUSTED middle-man. Someone with a spotless record and high buy/sell rating. Upon receival of money, the middle man informs the seller that the money has been received. The seller then sends out the item. Upon receival of the item to the buyer, the middleman sends the money to the seller.
As you can see, I think that seller fraud is a bit more important than flaking, but flakers can be given neutral feedback... This system will protect the buyer from seller fraud, give n00bs a chance to purchase items and not be turned down, and give sellers a reason to give neutral feedback on flakers...