Thanks for the replies everyone! The reason I wanted to stat up a Crossroads Demon is because one will interact with a PC soon. I wanted stats mainly so I could run a social encounter between the two. I doubt the PC will want to strike a soul trading deal at first, but I'm sure a few NPCs will over the course of the campaign.
I figure if the PC somehow wins the conflict (which should be incredibly hard), they can get a decent deal from the Demon - information or a useful trinket at the cost of some debt but without their soul on the line. That said, if I can semi-accurately model Crossroads Demons with DFRPG stats I'd be pretty stoked! Here's my shot, after reading your advice, researching some, and a bit o' brainstorming:
Crossroads Demon
High Concept: Demonic Genie
Aspects: Devil with the Details / An Offer That Can’t Be Refused / Sweet Lies & Half-Truths / Crossroads Contract
Skills
Superb (+5): Deceit, Empathy
Great (+4): Presence, Rapport, Resources
Good (+3): Conviction, Intimidation, Lore
Fair (+2): Alertness, Athletics, Discipline
Average (+1): Endurance, Fists, Might
Stunts
Honest Lies (Deceit)
Paragon of Sin (Conviction), as per Tower of Faith
Read the Surface (Empathy)
Powers
Crossroads Contract (Item of Power) [-0]
Sponsor (The Crossroads)*
Sponsored Magic (The Crossroads) [-4]
Supernatural Toughness [-4]
The Catch [+3] - Holy Objects and the like. (+1 for Rarity/+2 because it's well known)
Swift Transition [-2]
Items
Focii: Varies - This may be a quill, ruby red lipstick or similar (+2 Control/+1 Complexity to Wish Granting spells).
1 Enchanted Item (Power 3, Ex: Love Potion that applies 'Lust' aspect to a target)
Refresh: 0 (10-10)
Stress
Physical OOO(OOOO), Armor 2 / Mental OOOO, Armor 1 / Social OOOO, Armor 1
-------------------------------------------------------------
Sponsored Magic (the Crossroads):
In the Supernatural universe, the wish granting that Crossroads Demons do is a matter of spellcasting, instead of an innate ability, which limits what they can do**. Crossroads Magic mainly centers around Thaumaturgy (Mostly Entropy and Wish Granting), sometimes done at Evocation speed.
Extra Benefits: See Below
Crossroads Contract (Item of Power) [-0]
It is What It Is [-0]: A metaphysical contract between a Crossroads Demon and the oathgiver.
Unbreakable [-0]: Only a Crossroads Demon or its boss can destroy a Contract.
One-Time Discount [+0]: Having no physical form to interact with, it offers no discount.
Sponsor (The Crossroads) [-0]: As per the power, but it may only be used to fulfill the bargain*
*http://dfrpg-resources.wikispaces.com/Sponsor
**http://supernatural.wikia.com/wiki/Crossroad_Demon
-------------------------------------------------------------
With these stats, a Crossroads Demon has a variety of options to fulfill their client's needs. For a client who wants money or an object, the Crossroads Demon can make a Resources roll (at +4, plus some additional aspect Tags via Sponsorship to get what they need). If they want a task done, either acting on the client's behalf (murdering someone with magic) or providing an Enchanted Item (like a Potion of Love) might suffice.
For those who wants a little more, a Contract might be what they need - as it is an Item of Power the character is required to change/add an Aspect representing the deal which seems appropriate. For instance, if the client wanted to play Guitar better, they take the contract, add 'Bargain - Demonic Guitar Skills' and can now use their new Sponsor ship to take a debt and get a free tag on Performance rolls. Suddenly, they're a whole step more talented - if they were at +2/+3 (journeyman or professional capacity) they're now +4/+5 (veteran or masterful capacity) or if sent above that, they're now skirting the boundaries of human capacity.
Now, if the client wants something fantastic, like powers - that can be done via Sponsored Magic. As an Extra Benefit of the power, a Crossroads Demon can cast a spell akin to a Wizard's Deathcurse, using the willling client's life force to fuel the spell. The client doesn't pay the usual price of death instantly, but exactly 1 year later at which point the Demon will come collecting.
Besides that, the Stunts are there to make social encounters with them tough, unfair even (with Paragon of Sin, I imagine they recite to themselves some sort of Infernal Salesman Creed instead of a prayer). They should be able to have an idea of what their client wants and how desperately they want it (Read the Surface). And any time they can a Crossroads Demon should fulfill their end of the bargain with as little heavy lifting as possible (Honest Lies).
Supernatural Toughness is just a standard demon power, but Swift Transition is especially useful for just avoiding combat altogether (as long as they're caught in a Devil's Trap or similar!)
-------------------------------------------------------------
Well, that's my best shot for now! Comments, critiques, and suggestions are welcome. I'm sure a lot of fine-tuning can be done here :-)