Any clue what causes it to keep looping between mailing items and vendoring? 

Look in you're settings and check you're bags, if you dont have sell items on and but you have sell when you have only 2 free slots with you having less then 2 slots free in you're bag then it will do that.

6 minutes ago, The Smokie. said:

Look in you're settings and check you're bags, if you dont have sell items on and but you have sell when you have only 2 free slots with you having less then 2 slots free in you're bag then it will do that.

Can I make it ONLY sell when bags are full?

Change the value to 0 slots