So if I get this right, auction contracts allow you to sell something without going to the market. Unlike the market, you don’t need to be the same region, as long as you have assets in a station you can do this.
Item exchange contracts are to trade ISK for what you want to buy at a specific station.
And courier contracts is if you have assets in one system you need to move to another. With three characters, someone can jump into a Jita alt, buy stuff, switch to his main, do stuff in his main, and never have to leave his home system of his main.