I'd call the dealer you are shipping to and ask. Some FFL's won't take shipment from an individual even though it is legal in thier state.
Like YRU and quick stated, each state has their own rules, and sometimes even FFL dealers handle transactions slightly differently, even if the transaction is legal. It is legal in most states for you (an individual) to ship the receiver directly to an FFL dealer in another location (state). This saves you the cost of paying your FFL dealer to ship to the buyers FFL dealer. BUT, and it's a big but, a lot of dealers won't accept a receiver from an individual. It's a money making thing more than anything else.
Like YRU stated, call the dealer you are shipping it to. If they will accept a shipment from an individual, your good to go. If not, my recommendation, is to go on Gun Broker, and look for a list of FFL dealers at the location you are shipping it to that will. Notify the buyer, of what you plan on doing, maybe he can supply the name of an FFL dealer that will accept shipments from an individual.