You can find this guy in the West side of Varrock palace, ground floor
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If a player loses Darklight, it can be retrieved by talking to Sir Prysin in Varrock Castle and paying him 1,000 coins.
When you enter the Palace, you must look on your right. The first door on the right, inside that room is Sir Prysin. :)
Assuming that you used the bucket of water with the drain, the key should be here in the mud http://img502.imageshack.us/img502/6241/varrocksewers.jpg
Nope, it can't be traded or sold in the Grand Exchange. You can always get another one though for 500gp by talking to the Sir Prysin in the Varrock Palace.
It is just outside the inner castle wall, by the kitchen. The kitchen is in the north-eastern portion of the castle.