When I left click on the vendor his items show up on the screen - how do I get my items to show up on the screen in order to sell them? Thanks in advance

I personally use WASD + E to sell. Scroll down with A until you are in your character's items, and sell away.

("Y" also works quickly if there is a confirmation window)

Ya use wsad it's so much easier. You don't need to use your mouse in menus, e selects things.

There's a line that splits the shops items(on top) and your items (below the line).

There is a dividing line between the vendors items and you're items. Just scroll down main category list.