It's easy. All the big box stores have curbside pickup. If they reserved their ammo for curbside orders and limited the number of boxes they would sell per month to any given cell phone number, the situation would be much better managed. Local gun shops could do something similar. The problem is the retailers don't seem to really care, and thus they facilitate the gougers.

The retailers probably delude themselves into thinking that if customers have to come into the store to get ammo, they will buy additional products, but that doesn't work when gougers are buying all the ammo.