Abaut the kill for money thing: kinda like some have said, I probably wouldn't, but not for any moral reason only because I'm afraid for the law and I don't really want anything money can buy.
Abut selling my soul: this is an interesting one, generally,
it depends.
I could imagine making a deal in which the soul is duplicated 10 times, whit one copy going to eternal torment and the remaining 9 eternal perfection, thus I get 9 times as much good stuff than bad stuff, that all the numbers are infinities is not as relevant as it would seem to someone who doesn't know the math.
Another version of this would be to wish for another time dimension, the devil get one time line of soul (that has infinite length, thus an infinite amount of time) but as time is two dimensional even a hour
2 would have an area and thus be greater than an infinite line which does not have an area. I would have infinite area of time, or actually if the which was unlimited, n-dimensional time. If the devil cant make a new time dimension, even keeping time linear you could do something like letting the devil have the soul for ∞ seconds while afterwards it is restored to it's previous state (removing the memories and trauma of eternal suffering) and you get to live in perfection for ∞
2 millennia, or ∞
∞ gogolplex millennia, or some such.
summary; you don't HAVE to trick the devil, if you know some math you can get a deal which makes both happy. The key to defeating infinity is to know that there are things much larger than infinity.
Selling someone else's soul: I would, without hesitation.