I created a firestorm of hate at MetaFilter by inquiring about this. My thinking is that I have a service I can provide. Is it morally dubious? Yes. But I'm not making the decision. I'm providing a service. I realize that's sort of a convenient distinction to make, but I fundamentally believe in the moral autonomy of persons. I realize I'm inviting a cognitive dissonance into my life- I abhor plagiarism and I abhor paper buyers. But excusing myself from the ring on those grounds is hardly the "right thing".
Keep typing like that and maybe you'll eventually justify it.
It's not morally dubious--it's plain wrong for you and the person. They entered into a contract with their school to not cheat and you're aiding them in that. The fact that money is changing hands makes no difference. Whether it's a service or a favor, you're not in the right. You're hurting the person by allowing them to slide by and not work honestly, allowing them to slip morally. If they're competing with the class, then its even worse (depending on how well you write, I guess.) You and the buyer are both in the pits here.
I'm not saying I care and I'm not trying to reprimand you, but I just want it to be clear that this isn't a gray area. in my opinion, it feels better just to say "yeah, that was definitely wrong, wow." If you are fully aware, then you've both preserved your moral line and received what you wanted out of the situation