"Legal" for whom?
I was told by BATFE that an FFL dealer is "responsible" for firearms/ammo transactions conducted on his "licensed premises", which might include his parking lot if it is at the licensed address. That does not mean it is illegal, but it must be done in compliance with the rules, i.e., record keeping, waiting periods, etc., the same as if it was inside. (To use BATFE's example, a dealer could set up in a tent in his parking lot if it is part of his "licensed premises" as long as he obeyed all the rules regarding record keeping, etc.)
As to how a dealer could always see a transaction that did not involve him, or how he could be "responsible" for any actions not his own or those of an employee, I don't know; I guess a court would have to decide if BATFE decided to make an arrest.
I don't think "licensed premises" could be stretched to include a whole shopping center, but a parking lot or parking space clearly part of the address on the license could be interpreted that way. I think anyone actually needing information on specific cases should contact BATFE.
Jim