http://zqktlw5bf6oycxq6qod452xnvdfcqhfwliz54bkul736g6b454jrk2ad.onion/wiki/index.php?title=Physical_Interaction
Ideally, you should be using them for one-shot deals, like courier service or to set up an overseas account, or to open an account whose card and number will only be given via mail (ie to you, not them). The less information they have about what they are doing, and the less they see of the end result, the better off you are.