I have a used Shimano 6870 (Ultegra Di2) short cage rear derailleur available. Replaced for the large cage model 12 months ago as I wanted a 32-tooth rear cassette for a trip to the alps. Kept hold of it as I thought about building up another Di2 bike, but have decided against doing so now. So it's sitting there doing nothing.
The item would be conservatively described as "used" - it has some scratches and paint wear - but was in perfect working order when it was replaced. As it was a straight swap this only includes the derailleur - no cables etc.
New these are £140+ but I'm definitely open to offers.