Is there a fast way to sell large amounts of ammo?

There isn't a shortcut to sell huge quantities of ammo at once, but there are some small things to make it faster.

Holding the D-Pad on a controller will increase or decrease the volume of ammo sold pretty fast already. According to this Reddit post you can hold both the D-Pad button and move the thumbstick to double the rate that the ammo is transferred. This trick can be replicated on PC by holding both the D key and holding down the transfer arrow using the mouse.

You still won't be able to sell stacks of ammo without stopping to select a different stack, but it should make it slightly less painful when you're trying to get rid of 2000 bullets.