A user-to-user license transfer is by disabling the seller api and enabling the buyer.
Always a good idea to:
1.- verify the seller is legit by PMing me
2.- use an escrow

By the moment the transfer is complete, the buyer would be like another normal customer of mines, with support/upgrades etc.

Comparing CAT and GUNBOT is not doable imo: both do 2 different things. They have 2 common features tho: they both are very profitable and both their devs are italian 

Hope it helped.
~Gun