I usually just steal the skillbooks my party needs, and whatever else I can grab to push that max value bar up. NPCs restock when you level up or after some time has passed (not sure the exact time, but possibly 60+ minutes real time).
The higher level you are, the better stuff they sell. Most of the skillbooks worth getting don't appear until you hit level 4. I usually wait to pickpocket until then, since you'll usually want these new skillbooks, and they can get pricey.
Oh, and a tip for stealing: Have a party member talk to the NPC, then sneak/pickpocket them while they're occupied. Then cheese it out of there. Make sure the character talking to the NPC doesn't have any stolen goods on them, because they'll usually be searched before they can get away.
Last edited by LaughingLeader; 11/06/17 12:28 AM.