Christie’s Auction House in London is auctioning off what they believe to be the Empire Strikes Back Vader costume on November 25th and it’s expected to go for anywhere between $250,000 and $365,000. They believe it is from Empire Strikes Back, and do not say who donated it.
Before you go and bid, you really should track down a Vader from the 501st and have them take a look at the suit. There was a slightly different Vader used for each Star Wars movie, and no self-respecting 501st Vader would dare not know the various minor changes made between the suits.
If you’re curious about the changes made to Vader over the years, here’s an excellent resource all about it.