Does Half.com charge for refunds (Note: I am the seller)?

Probably because of the credit card transaction costs money. Both ways the transactions will have to be taken by the middle man. In some cases the company will take on the costs, in some cases they will make the seller accept the costs. Either way its not the same as giving cash, there is no middle man in such cases. But when it comes to online purchases and any credit card transactions, the middle man will be the bank or card company. They don't care if its a refund or not. They still have the process the requests to their computers, which costs them money. So someone has to take on the costs.