A merchant must have the following components:
Your room title will be the name of the store. Typically this would be something like:
A Silvery Tent, Janella's Jewelry or Janella's Jewelry, Show Room
You can do it either way, describing the type of building the shop is, or just describing the shop itself with your title.
Your room description should reflect what the inside of the shop looks like. This will be what players read to find out where items are in the shop to look at. If items are going to be on things like racks or tables, make sure you mention them in the description. If you need help writing room descriptions, please see the article How to Write Rooms.
Once you get around to describing the things your merchant sells, you're pretty much following our standard alteration guidelines and quality control rules. If you need help writing alterations, please see the article Writing Alterations or Custom Items. Your merchant can have as many items as you'd like, but do keep in mind that it is a shop, so you'll want to have a good number of things available that other players can purchase.
Those are the basic things a merchant needs to have. You can also so so far as to include verbs or atmospheric messages for the items you're designing. You can also include an atmospheric message or two for the shop itself. For example, a faint scent of vanilla might drift through the air, or the merchant may have some incense burning, or store attendants may be going around and straightening things.