At his shop, it is where you can buy and sell off items. He will also inform you of important locations throughout your game. The store is open from 8AM-11PM. By 10:50PM, you will start to hear a closing BGM which signals the store closing.
To shop for goods, simply walk up to the item, directly face it then tap A or tap the stylus twice. Normally, goods will change to a "sold out"status for a day after your purchase. To sell items, talk to Tom Nook and choose sell. Transfer the items you want to sell to Tom's bluescreen and he will provide you a selling price for you to decide. Usually the selling price is 25% of the original price from Tom's store.
Another important thing Tom can do for you is that he can tell you the current turnip prices. These turnips are priced according to the "stalk market" pricing. This price changes twice every 24 hours during noontime. Bear in mind that on Sunday there is no trade available and the turnips spoil after 7 days.
Tom sells different goods everyday so you have to check out his store everyday. Also the store becomes larger as you transact more business with him, giving away more available items in the process.
Construction for Tom's shop upgrades takes 24 hours to complete and you will be informed by notices posted on the Message Board by Tom Nook himself.
Below are the upgrades for his shop and details on them.
UPGRADE: Nook's Cranny
ACQUISITION: Start of the game
Tiny building. Has 1 carpet, 1 wallpaper, 2 furniture, 1 package of seeds, 2 tools, 1 medicine bottle, 1 message in a bottle.
UPGRADE: Nook n' Go
ACQUISITION: Transact 25,000 worth of items
Convenience Store. Has 1 carpet, 1 wallpaper, 3 furniture, 2 packages of seeds, 2 stationary packets, 3 tools, 1 medicine bottle, 1 message in a bottle, 1 sapling.
ACQUISITION: Transact 90,000 worth of items
Supermarket. Has 2 carpets, 2 wallpapers, 3 furniture, 4 packages of seeds, 3 stationary packets, 4 tools, 1 medicine bottle, 1 message in a bottle, 2 saplings.
ACQUISITION: Transact 240,000 worth of items, have a visitor buy or sell 1 item
Department Store with 2 floors, also houses Shampoodle Salon and has the twins Tommy and Timmy Nookling working on the second floor. Has 3 carpets, 3 wallpapers, 8 furniture, 7 packages of seeds, 4 stationary packets, 6 tools, 1 medicine bottle, 1 message in a bottle, 2 saplings, 1 cedar sapling, 1 paint can.
There is a small machine in the shop called the Point-Tracking Machine or PTM. For every 100 bells you spend to purchase from his store, you will receive a point, and 5 points on a special day. Discounts get better as points go up and eventually you'll get a rare model item.
PTM Points: 300
Item Reward: Nook's Cranny Model
Discount Given: Store Membership
PTM Points: 5000
Item Reward: Nook n' Go Model
Discount Given: 5% discount
PTM Points: 10000
Item Reward: Nookway Model
Discount Given: 10% discount
PTM Points: 20000
Item Reward: Nookington's Model
Discount Given: 20% discount
The personal catalog lists down all of the items you have acquired that are sometimes cannot be found in the store. This is especially useful if you are tracking down an item you wanted to keep but somehow sold it by accident. It also shows a view of furniture items which can be rotated so you can visualize how it would look like in your house. Catalog orders gets delivered to your house within the day or the next day. You have to have your gates open so these orders can be delivered to your home.
Seldomly, Tom Nook will reveal a 'spotlight item'. They are limited items which are only available during the spotlight timeframe so purchase them asap if you have the need for it. Some of them can be found in sets, while some are not. Even some spotlight items can be found in some areas like in the Lost and Found, Recycle Bin, etc. or through various other ways. It takes awhile before another spotlight item pops up so don't waste any opportunity you can to get that item.
There are workarounds however, to increase the probability of a spotlight item showing up. One is to to increase your membership level. Another is one is to increase the amount of human player characters in your town.
He will also sell limited-time seasonal goods depending on a particular time of the year. Shipments of special holiday goods only appear during that specific time of the year, and you have to wait until another year for them to show up again.
Take note as well, that sometimes Tom will sell all of his products at a 50% discount. This sale could happen from 5PM to 8PM and will be notified by Tom Nook on the Message Board.