Creating a new vendor

1. Create a new NPC.
2. Create a Quest, tick start game enabled, enter a quest ID and then click "Ok", then open the quest again.
3. Go into the topics tab of your quest, right click the white list on the left, click add topic, scroll down to "GREETING"
4. Also, create a new topic by right clicking on the list where you found "GREETING"
5. Right click the info tab of your GREETING and click new, make the response text whatever you want, this is what he'll say when you first talk to him. (Leave the prompt clear)
6. This is very important, right click the "conditions" tab and select new, click the "INVALID" button and scroll down to your NPC's ID (Make it == 1). This makes it so that only your NPC will use the custom greeting you created.
   1. Go to the new topic you created earlier, by using the same methods you used to create the GREETING.
   2. (right click the "conditions" tab and select new, click the "INVALID" button and scroll down to your NPC's ID (Make it == 1)
   3. Response text: (This is what he'll say when you click the "Buy" Button)
   4. Prompt: (Whatever you want it to be) Example: I'd like to buy something.
   5. Now, at the bottom in the "Result Script (Begin)" box type: SHOWBARTERMENU
7. Create a NEW (Meaning new ID etc.) container, check off the Persistent Reference Box. Here's the tough part: finding a .NIF file for the model. You will need to get the FO3 Archive Utility and open the Meshes.bsa file. Once you have done that, find a container you want the merchant container to look like and select it. Then extract the file you selected to the Data folder. Now you are ready to add the .NIF file. Double click on your container in the Object Window and click on the edit button and navigate the folders until you find the mesh you selected earlier. Once that's done, you are ready to put the container inside your store's cell. A good place to put it is somewhere the player won't be able to gain access to it. If you do choose to place it inside the store itself, then be sure the container is locked. Then put whatever you want in it, this is what the merchant will sell. Be sure to put some caps in it so the vendor has some money to buy some of your things.
8. Place your NPC inside your store, double click him in the render window and select merchant container, and scroll down to your container's ID, click okay, save and you're done.
9. Go to you quest's topic tab again and click the GREETING, go over to the "Add Topic" tab, right click it, select Add Topic and select your "Buy" topic ID.

Optional

Phew, now, if you want to talk to him through an intercom.

1. Go to the Talking Activator Tab under "actors" and create a new one (New ID) and put it in your house.
2. Double click your NPC inside the store, at the top type a new ID at the top where it says "Reference Editor ID"
3. Create a new script:

ScriptName scriptID

Begin OnActivate
    setTalkingActivatorActor VendorReferenceID
    activate
End

and attach the script to your talking activator