There are a number of options we could look at. The two largest that spring to mind are what the United Kingdom has, something known as UKARA and UKAPA. They are two associations that work with each other, UKARA being retailer-based and UKAPA being the player version of that.
In order to purchase airsoft guns, you would need to be a member of UKAPA. In order to sell them, you would have to be a member of UKARA. There is also the Netherlands, which uses a number of local associations to effectively do the same thing. The associations would be responsible for maintaining records, as well as for an age verification system to ensure that those purchasing the firearms are 18 or older.