If you buy a low level item from a vendor (for example, Geras in Shattrath City, or the various armor token vendors in ICC), Vendor will try to sell those items back to a vendor before the 2 hour refund window is over. This generates confusion on the part of the addon, as it will say that it sold the items for 1 copper, without actually selling the item.

While this doesn't generate an error that BugGrabber will pick up, you do get red text from WoW saying there is an "Internal Bag Error".

This doesn't create a functional problem (as far as I can tell, nothing really happens, and no items are actually sold).

Oh interesting. I think the scenario here is that you have an item that evaluates to sell, but it isn't purchased with gold, it is instead purchased with some other currency, and so there's a 2 hour refund window where you can get the currency back.

I think I understand what is going on here, I'll add this to our list of things to fix for next release.