There is an important difference between PvP combat and PvP pickpocketing, even if consent is explicitly required: in PvP combat you can fight back, in a "PvP pickpocketing" the pick-pocketed player is always a victim (not much fun).
When trying to pick someone's pockets you can succeed, in which case the pick-pocketed player is the victim, or fail, in which case the pick-pocketed player can retaliate with whatever means they see fit, including (if it can be implemented) calling the guards to ban you from the city, or starting a PvP attack (which the pickpocket should in my opinion not be given the option to decline.)
Can't players use dice, Ã la Dungeons and Dragons? Like, there can be a dice system for this kind of RP actions and detections.
That is what is currently done but, unlike D&D, there is no way to measure your own resistance to pickpocketing. I presume a pickpocket would easily detect someone trying to pick his own pockets, right? My idea was the game would be able to better handle such metrics, by some rule that would take into account both parties' stats.
In any case these are just my personal views on a possible implementation.