Go to Shop ---> Trade.
I don't really remember if Meredith will offer to sell you a Trading Pass just because you don't have it yet, but she WILL offer you one when you try to start a trade with anyone.
When it asks for a password you're supposed to type in your own Gaia password.
A Vending License was needed for the old Marketplace, months ago. You could only sell items when you had that license, but they removed it when they set up the 2% fee for selling an item.