That's what multi-signature escrow is for. You deposit money into an account and you have one key. The vendor has one key and a third key is given to an arbitrator and at least two keys must sign the transaction in order for it to be completed. If the vendor sends you your product and you are happy with it, you sign the transaction and the vendor signs the transaction and the vendor gets the money. If the vendor does not send you what you ordered, you talk to the arbitrator and the arbitrator listens to your case and the vendor's case and makes a decision.