I second the vote for reading the Dresden Files to gain inspiration, but if you need the shortcut here it is:
The Fey (all of them) are old, cunning, intelligent and never lie but never give a straight answer as well. they offer bargains to ensnare unwary people. they ofer things like knowledge, power, riches, food, carnal knowledge, what have you. Remember Wish in 3.5? With the fact that the DM should properly screw you over if you wouldn't ask for a standard wish and didn't word it carefully? That's what the fey do. the only difference between seelie and unseelie (and wyld fey) is the fact that the season they represent might influence what form you get your bargain in. Seelie are generally nicer and ensnare you with compliments and basically making you want them. the unseelie fey will make you miserable and make you need them. What do they want? they want your soul. But lesser bargains apply: a certain amount of memories, a sense, your firstborn, they aren't really picky as long as it can be used to screw you over or lure you in enough to completely win you over.
While they can not ever speak lies they can discuss probabilities and possible consequences. When a bargain is struck don't expect them to keep to the spirit of the bargain, but to the exact letter (including interpunction). A verbal contract is more then enough (they can't break it) and above all they WILL get what was promised to them.
the funny thing is, in DnD they are represented as being chaotic, but in mythology and the Dresden files (which IMO does a great job of describing) they out lawyer the forces of hell. they will absolutely keep to the letter of the law, not the intention, no matter what.