Looking for classes revolving around soul selling?

[TinyMushroom]

So I'm currently building a character whose whole premise kinda revolves around selling his soul for power. Originally I was going to build a Warlock, but I'm afraid I might get bored of the simple mechanics of the class. Is there any other classes out there people could recommend that revolve around the same premise?

[Glimbur]

Sorcerer is a decent fit. Ur-Priest could be reflavored to work, though you'd need a base class first. Binder is a different possibility; you could even re-fluff Incarnum if you really wanted to.

[Zaq]

A Binder (Tome of Magic) doesn't sell their soul so much as lease it out. You make pacts with vestiges (mysterious entities separated from the fabric of the multiverse), allowing them to cohabit your soul for 24 hours, as they desperately want to feel some connection to proper existence again. You make a check that determines whether you made a good pact or a bad pact. You always get to use their powers, regardless of the result of the check, but if you make a bad pact, they get to exert their influence over you, which usually takes the form of a list of requirements and/or prohibitions on your behavior (with stacking penalties if you choose to disobey them). It's got some really interesting mechanics, and since you can change which vestige or vestiges you bind every day, it's a lot harder to get bored with it than with a Warlock. I think it gets a lot more interesting once you get the ability to bind multiple vestiges at once (since when you only have one vestige at a time, it's kind of hard to cover both in-combat and out-of-combat tricks at once), but it's still a fun class even from level 1. I highly recommend you check it out.

It's not quite as blatantly selling your soul as you might get from a Warlock, but in some ways it's even more interesting.

[The Viscount]

If you take the warlock route, Demonbinder could easily be centered around selling your soul, since it's all about demons. Not to be confused with Nar Demonbinder, which is very powerful, though generally too clever to sell their souls, though still definitely in the demon summoning game. Malconvoker could work really well, especially if you lift the nonevil requirement or play a neutral closer to evil.

[Rizban]

Not a class, but the Fiendish Codex II has Faustian Pacts where you do, in fact, sell your soul in return for benefits.

[Valwyn]

Diabolists from BoVD sell their souls to powerful devils for power, but their souls are only collected when they die. (Does this mean you could cheat them out of your soul by becoming a vampire or lich?) It seems like a rather weak class overall, though. Other classes in the BoVD might fit, but I think Diabolists are the only ones who outright sell their souls. Lifedrinkers gain power from bestowing negative levels, but it forces you to play a vampire.

Speaking of vampires, a feat in Libris Mortis called Necrotic Reserve lets you survive being reduced to 0 HP if you drain someone's energy first (life energy is a soul thingy, right?)

A character with thinaun weapons (CW) could capture souls in daggers and sell them. I could imagine an assassin getting into that sort of business.

A high level wizard/sorcerer could use Trap the Soul (conjuration 8th) or Soul Bind (necromancy 9th) to capture souls.

That's all I can think of. Other than that, you could simply refluff other classes. Each time a wizard levels up he needs to sacrifice someone to gain his two free spells per level. Or a cleric gains new spell llevels by earning/buying souls from his boss. A blackguard could barter with his demon companion. It's up to you.