I was re-reading a favorite book (Twilight Falling) and this line caught my attention. The protagonist needed to keep a protective shield on an item of importance but his shield had been "spent" for the day. His companion, a thief, happened to have one available & used it. Which brings me to a possible Thieves Guild ability: the ability to shield vendors from stolen swag (which means the vendor would then buy said stolen items).

Just throwing it against the wall. Happy hunting.