I think the GunBroiker owners are cashing out to avoid the risk of adverse legislation that would stop the online sales of firearms. Selling the site for many tens of millions of dollars will make for a nice retirement. I watched the GunBroker presence at the SHOT Show grow year after year, indicating rapid growth - and value increase.
The site makes a LOT of money, making the site worth a LOT of money. The current surge in demand has boosted selling prices, which, in turn, generate higher final value fees, making the company worth even more. Ammo is willing to bet that the database is worth the price paid, and that legislation will not destroy the value. Ammo will sell the majority of its ammunition on the site, thus increasing profit for Ammo.