If you can't find an item on the Shop Wizard, try searching for it at the trading post or auction genie -- just in case you managed to snag a really rare item that's worth more than you can charge in your shop (which is 99,999 NP). If you do have a rare item to sell, we can help you with using either the auctions or trading post.
Here are three good shops to try:
Igloo Garage Sale, which does have some overpriced items. Ice lollies (not brucicles), codestones, neggs, A Mysterious Valentines Card, most collectable cards, most books, and some stamps will earn a profit. Avoid furniture, toys, candy, etc., unless you know it's a good deal.
Second Hand Shoppe, in which everything is free! Avoid the dung. There are a few things that only sell for 1NP, but once you learn those, you can try for the rarer, more valuable items. For both IGS and SHS, there's a daily limit on how much you can snag -- it used to be 10 in each, but I think IGS's has gone down (maybe to 5?). IGS will tell you when you've reached the limit; I don't think SHS does (I very rarely make the limit in either, so I'm not sure).
Collectable Sea Shells, in which there are only about 3 or 4 items that do
not turn a profit. It doesn't restock in huge batches, but not a lot of people restock there, so I think it's a good shop for beginners.
If you like games at all, KeyQuest gives out item prizes that you can sell in your shop. So does Dice-a-Roo. And then there are a number of free (or pay-for-themselves) daily activities that also give items.