you base the rates on the price of where it's sent and stipulate that in the user agreement? say it costs $5 for me to get a certain weight item from the US to the UK then that should be the price that the seller adds. By buying it at that price of postage we agree to the terms? Or you could have it more decentralized and unregistered operate in secret. Depends who is the one setting it up. Some people are like fuck the government for example. Also adding an escrow system. That mitigates alot of the risk. Also postal tracking is always a good idea.