A white elephant is a rare kind of elephant. In Myanmar two have been found and caught, the second one in 2002 after an elephant finding team had searched for one month .
In Thailand, white elephants are sacred and a symbol of royal power; all those discovered are presented to the king (usually this is ceremonial -- they are not taken into captivity) and the more white elephants the king has, the greater his standing. The current king Bhumibol Adulyadej owns ten, a great achievement, since three is considered great. This is probably more due to modern communications than anything else, though considering the creatures' rareity in other countries, it is still impressive. In the past, white elephants have been given as gifts to the king's friends and allies. The animals needed a lot of taking care of and, being sacred, could not be put to work, so to be taken care of, only the monarch (or the very rich) could afford them, so were a great financial burden on the recipient.
Because of this, the term "white elephant" came, in English, to mean a thing which is more trouble than it is worth, or has outlived its usefulness, to the person who has it. While the item may be useful to others, its current owner would usually be glad to be rid of it. By reason of this, commercially, a "white elephant" might be available to purchase at a very favorable price.
An example of such an item might be a large, old house, occupied by an elderly widow who has neither the physical prowess nor the funds to keep it up any longer. The widow might sell the home at well below its ordinary market value because it is a burden to her. The buyer, likely someone who has use for a large home, would get a good bargain.
Hence a "white elephant stall" is a stall of bargains.
According to one story, white elephants were sometimes given as a present to some enemy (often a lesser noble with whom the king was displeased). The unfortunate recipient, unable to make any profit of it, and obliged to take care of it, would suffer bankruptcy and might even die.