Disney does a couple of things with items that do not sell for MSRP:
1) Offers to a Disney Outlet Store (which are independently owned)
2) Churns larger items to other Disney parks (i.e. Euro Disney, Tokyo Disney)
3) (And this is the most likely) Sends them to "Property Control" and sells them to the Disney Cast Members at a deep discount price. They also sell furniture from re-decorated rooms, golf carts, store displays, lost & found items, you name it, they sell it at Property Control. I worked for Disney in 2003 & got in on some truly remarkable sales at "PC".
For those of you visiting WDW this Christmas season, you'll see tents set up in the parking lots at Epcot & Port Orleans Resort. These are two additional sales areas for PC. The main one is behind the MK.
The days of sales apparently are over. Once in awhile you'll see tee shirts on sale, but that's about it anymore.
A lot of campers leave things that are cheap to purchase but a pain to pack & take with you such as.... cheap tents, ground cover carpets, cheap screen rooms and the list goes on. If they are servicable, they go to PC to be sold. Disney never misses an opportunity to make a buck!!