Shops allow you to buy and sell things that you need. The implementation of shops is very different from NWN2 normal as is the money.
To buy things you should go to the shop, look through the shop containers in form of chests, shelfs and so on. The items have a price tag.
You can then pick up what you are interested in and if equippable freely try them on.
When you find something you wish to buy, you should put back the rest of the items you picked up to the place you took them from as shopkeepers do not like you dropping them on the floor.
Actual buying is done by talking to the shopkeeper asking to buy the things.
When you buy something, the final buying price is modified by your appraise skill.
You cannot exit a shop carrying unpaid merchandise.
Selling works by going to the shop and placing items you wish to see in the merchants desk.
Then to do actual selling, talk to the shopkeeper who will tell you what he offers for them. You can then sell or not sell.
The price s modified by your appraise skill.
When you have acquired loot from adventuring, you can also sell them to most shopkeepers. Selling such is done by just going to a shopkeeper and telling them you want to sell some of your loot.
Some of the special stores, like the herb shop and the mage guild study material shop allow you to buy special things if you fulfill some conditions. In general the condition is helping the shopkeeper first, and even then the amount you can buy is limited by how much you have helped the shopkeeper or a guild rank.