The way that the dealers work is that an admin or server owner must select a door on the map (any one of your choosing) and do /newdealer I believe is the command. It took me a while to figure it out as well. When you do this command, it will ask you what times the player will be able to interact with the door. Now the way that this works is that you need to do the missions for the dealer to get rep. The more rep that you have the more that you can buy from the dealer.
BTW: You can have as many dealers as you want I believe. And the rep should be shared between them.
That one I am not too sure about. I believe that you can only do the deliveries within the time window that you specified for the dealer. But it should display a blip for you to deliver to. Otherwise it would just be one of the locations listed in the Config.