They take the risk and hope ebay dont spot the auction.
Edit, Yes, ebay considers paintball guns to be firearms.
As a seller on ebay of paintball upgrades, Im reminded of ebays firearms policy every time I list.
In the UK paintball guns are classed as toys.
But in some countries paintball guns are classed as firearms.
And in any case, if you modify a paintgun in a certain way, and/or use it in a certain way, it is a firearm.
Ebay just dont want to get involved in that.