It's not a matter of easy or hard, anyway. It's the fact that having the vendor's inventory "physicalized" in his pockets is WAY too prone to abuse, period.
There could be 2000 attempts to retune and balance it, make it harder or whatever but simply none would be as simple and effective as having a separate "pocket inventory" for pickpocket, with a limited availability of options.
BG, BG2, IWD and all the Infinity Engine games used this expedient and it... Just worked. You could even pickpocket "celebrities" eventually and get some powerful little gadget (magic rings etc) but you were never allowed to loot entire stocks from the back pocket, which limited the potential abuse.
