Whether a criminal buys a gun in a parking lot or at Wal-Mart or online is not really a debatable issue. Criminals are going to get a gun wherever they can find someone to sell them one or where they can steal one. I know legitimate gun owners, who have no intention of mass shootings, that have bought guns from collectors in a walmart parking lot. Where the gun is purchased doesn't matter. Odds are a criminal is going to steal the gun rather than put out the cash to buy one in the first place.
While it is convenient to go to a gun store to purchases these weapons, there are a few reasons I could think of that would compel someone to purchase them from a stranger in a parking lot. For example, if they buy it from a stranger, there is no paperwork and no background checks. This could make someone feel it would be better to buy the gun in a parking lot instead.
If someone is that desperate to get a gun, they are not going to be concerned about legal checks or buying from authorized dealers. Criminals would certainly go to a parking lot to buy a gun if they could be sure they would get cheap guns there without background checks.
I suppose it COULD be a parking lot - if that's where they meet the person from which they are illegally purchasing a gun. I think the Internet also facilitates gun purchases, as well as gun shows. Criminals with agendas will definitely find a way to obtain weapons to follow through with any plan they may have, though - no matter where they are sold.