She was the new apprentice to a Jedi knight, only at the very beginning of her training. She was eager for her first mission, but had to watch as it turned into a complete disaster. Their ship was ambushed by pirates, her master was killed, and she was captured. Recognizing the value a Jedi, even an untrained one, would have on the market, they brought her to a fringe trading outpost to be sold as a slave.
As fate would have it, a young sith apprentice was on the station hunting a fugitive. Sensing an unexpected presence in the force, he became curious and went to investigate. When he saw the Jedi on the block, her potential was obvious. She could be a powerful status symbol, and if she could be turned, she would be a powerful weapon against his enemies. He slipped into the crowd and started bidding.
I'm looking to play out the unusual relationship between the two. She might start out as a simple captive, but over time will progress to apprentice and lover.