I think most people do this already.
Some factions go as far as RPing actually helping the NPC chop the vehicle
-1 to collecting money from the npc.
Finding vehicles to chop without killing someone for their vehicle has become complicated enough at times unless you're a member of a gang.
Having to collect the money from an NPC just gives the victims/PD a chance to camp the NPC.
If I brought a stolen car to a back alley mechanic he isn't going to admit he's taken a stolen car to a member of PD or the victim it was stolen from
