You buy stuff from general stores that are rare when they come out every so often, hard to do with a slow computer though. :| They go quick.

Restocking would get you up to a high amount of NP.

Once you reach 50-100m you can start reselling though. Buy low/Sell high. Also Auction sniping is an alternative

Reselling items is profitable, but it's a patience game for the real money, but really you're a Walmart at that point. It's not about the profit from a single item, it's the pure volume that makes you the profit.