UI glitch
 

I've been getting this glitch for a while. I can't seem to get rid of it permanently. It shows a window with sliders and settings in the upper left of my screen. If I reloadui it will usually go away, but it comes back randomly when I log back in some days. I can't tell what add-on it is. I suspected it was AwesomeGuildStore or maybe Arkadius Trade Tools. Anyone have any ideas?

Ideas: No.
But you can simply test it by
ceating a list of addons you are using as the error happens.
You could use Circonians Addon Selector to create a profile for testing e.g.

Then disable addons that got to do with the listings etc. one after another and check if the error happens again.
This way you are able to determine which addon it is.

Or:
Enter this script in your chat box but do not press the return key yet:
Move your mouse over the controls that show up, e.g. that editbox with the 30.
Then press the return key in the chat having your mouse above this 30 control!

It should write the name of the control to the chat so you are able to see the name of the addon somehow in that name.
If it's only GUI or GUI_ROOT try to move the mouse above some other control shown there, as GUI is only the UI where ALL addons and vanilla stuff adds to.