Vouches should be able to be given and received for graphics that money was paid for. It takes a lot of trust to know that your artist will fix anything wrong with your graphics if you don't like it after the payment is done. Like anything else, graphics is a service provided by a user, in which you trust to send them money. Same goes the other way around, if someone doesn't charge me back, I should be able to vouch for them after a certain period of time.