Hi,

Is there an addon that will automatically purchase items from a vendor without you actually being there. As a leatherworking I buy a lot of salt and thread and hate going back and buying them over and over.

nope

Entirely not possible. Everything in the game requires some kind of user interaction.

No, you have to actually GO to a vender to buy something from him/her. However, there are a lot of mods that will allow you to automatically restock your bags every time you visit a vender that sells things you need. One particularly nice one is Reagent Restocker. It also allows you to refill your bags from a stock of salt, thread, ect. that you keep in your bank.