So, since the update, I can't sell to vendors. There is no indication of how to swap from the Buying screen to the Selling screen. I'm playing on a PC, using a PS3 controller. The closest thing I see is an option to "Change Panels" by hitting the Wait (aka Pause or Select) button, but nothing happens when I press it.

Anyone have any idea? I'd like to be able to sell some of this junk.

I can't say on the PS controller, but with the 360 controller I use the "back" button to switch from buying to selling... Which is the small button to the left of the "power" button.