1: First, make it so the players aren't metagaming, if the characters have no reason to fear the fey, have them accept food just like if a commoner or other adventuring party offered, yes, it may result in a TPK, but that is the fey. And it leads to trying to release to person from say, an eternity of dancing. You could also give them a NPC whos job is just to accept the first offer while the PC's don't in order for the group to see the possible consiquences.
Speaking of the effects of seeing what happens if they would have accepted, I remember an excerpt from something, heres a rough qoute, embelished at the end.
However, if they keep refusing, have the fey become slowly hostile, until they eject the players from their realm.They lured me to the dance, sweet smeels tantilised my nose and satyrs music of the sweetest of kind lingered in my ears. A buety without compare smiled at me and waved a hand at me in a universal come hither motion.
"Come Dance With Me."
With a great test will, I refused.
Her face turned mournful, then turned as she kept dancing.
Then I saw it.
The music turned sour, a dirge at high speed, the buety was just a head atop of rotting bones, the dancers where jerky, dripping with sweat.
I could swear I heard one say.
Those with high will saves and enchantment resistance will obviously fare much better.
2: Consequences, make them up, your the DM and as said, it is definately out of the scope of the rules and thy have their own realm, the fey are supposed to be hugely powerful, but have some weird things that stop them dead, like counting grains of sugar 1 by 1. While I don't know the 4e rules, things like wish and geas are entirely within the scope of their powers with their promises. However, one must note that things have a different value to the fey, a twig or acorn could be worth their equivilant of millions, but a pile of gold is just a useless pile of junk.
A trip to the fey realm should only be MARGINALY more suvivable than one to the far realm. There is a reason druids worship nature, not the fey, they simply co-exist with them.
Edit: Also, in regards to what sort of deals, there are fey warlocks. Perhaps if they impress one....