I usually check at the bottom of the homepage at the "Who's Online" to make sure I'm buying NP$ from someone who's currently online if I need them in a hurry. If they're not online, it could end up taking 24-48 hours. I've never had it take more than that.
As for a buyer not paying, hasn't ever happened to me, however I guess you could report that to NP Management so they could return the NP$ to your account.