See, it's the social aspect I was aiming for. What sorts of bargains should the Fey offer? I like the suggestions for prices: senses, small acts of evil. What should the Fey offer in return?
The reciprocity idea is a nice one. I feel like the party will be quite wary of accepting gifts, especially food. Is there a good way to highlight what would have happened had they accepted, to make them feel more of a rush when they refuse gifts?
For those suggesting mechanical ideas, this is actually for 4e, but I'm mostly focused here on bargains and tricks outside the scope of the rules, and not simply ways the fey can harass the PCs as combatants.
As players, have you ever made a deal with the Fey, in one game or another? What were the consequences?