I once (now about 5 years ago) sold a pc to a coworker of mine. I didn't even make any money out of it because I did it as a favour. I wish I had. Little did I know that she was the mother of all misers. Up to this date, she's still constantly pestering me with this and thats. I try to postpone going to her house to fix it, sometimes for months, but she's very persistent.
She never pays anything, and my incentive to do her favours is weary very VERY thin!

Now I wouldn't even sell her a PC anymore, not even with a $1000 profit margin! If she ever contacts me because she wants a new pc (which I expect any day now, since her pc is now more than 5 years old), I'll point her towards the Dell site for sure.