Using the same housebuild system that Player Homes use, the Player Shop system allows players to own their own shops that sell items or pets (but not both).
The shop consists of a stock-room that only shop owners can enter where the items or pets are placed on the ground to be sold. It also features an NPC Shopkeeper who will sell the items/pets for you.
Your character will receive a note when a sale has been made and the money will be placed on the ground inside the stock-room.
Pet shops may hold 20 pets at a time and item shops may hold 50 unique items at a time. For item shops you may use the ADDSURPLUS command to fill in a an extra stock of up to 50 of the same item without using up the store's item limit.
Shop Discounts allow you to charge more for some people and less for others. A popular way to do this is to flag newbies with a discount, for players under level 30, and jack the price up for players from clans you don't like, while giving a discount to your own clan.
In short, the Player Shop system is an involved one and for further information you should consult the Player Shops Helpfile.