A "white elephant" is used to describe something that is useless to you, but at the same time it costs a lot of money or effort to have around, and you can't ignore it.
I think it came from an old story about an Indian prince who had a white elephant that he couldn't put to work (not sure why), but he still had to feed it a lot of food. And because it was an elephant, it was difficult to ignore. ;)
A "white elephant sale" is another name for a junk sale where people could sell off the things they don't need anymore.