There are some downfalls that I think you have overlooked...
Before I say anymore though, I'm not saying don't go for this. I'm sure a load of people would really appreciate it. Personally, I never cross a warranty (unless absolutely necessary or if it is 110% beneficial to me), I just don't see why you'd want to give up something that's free. But I do just want to help you see what you might have overlooked.
And for those bits, even if you don't do something wrong to the console, then they can still get you for it (refund/lawsuit, yeah that's extreme/threat/etc) if something turns up wrong. Also, shipping and handling isn't perfect... what if something goes wrong there? I know I'd be pissed if something I shipped off to someone came back with a massive dent in it somewhere. If something does happen to these consoles, you would promptly (most likely) have to buy them a brand new console. Even if things aren't your fault... someone could still be a major dick.
I think that the idea is excellent, and that anything you can do/repair/service/etc on your own and make a profit off of is well worth it. I used to set up people networks for them back in the day! Got paid quite a bit from some for doing it too... (also used to mow lawns back in the day as well).
I guess what you really need to do is be extremely systematic with it and record/document (with pictures if possible) what issues the console had, what things the console needed to be fixed with, state and time that the console had arrived to you, etc. And then finally document/record the things that you have done to repair the console and how the condition of the console has differed since you have received it.
All in all, I wish you luck with it bud, its gonna be hard to start this one up.
I will always be around for anyone who does have hardware/software questions on QsK and if there is something that needs a walk through, I've always got skype
