The Repo Reaper talent immediately refills 30 rounds into your ammo reserves upon killing any enemy with direct damage from a sidearm. The talent will not give any rounds above the ammo reserve maximum, any excess rounds are lost.
As the talent always refunds up to 30 rounds, regardless of ammo type, it is much more beneficial to have a weapon with a small ammo reserve such as Marksman Rifles or Shotguns in the primary weapon slot, as the talent can completely refill the ammo supplies in a handful of kills.
Repo Reaper will only trigger when an enemy is killed using direct damage from the sidearm. Damage over time effects, such as Bleed from the Damascus or Burn from Incendiary Ammo will not trigger the effect. Also, using a Double Barrel Sawed Off as a sidearm will currently not trigger the talent, although this may be a bug.