You know... You could have checked whether it was just a screen defect.
(Without opening... If the machine actually boots up fine, log in blindly and try to hit key combinations to operate the mac... play some music, I don't know... Connect an external display or try to VNC into your Mac. (hell, even SSHing would do the job))
This way, the buyer at least knows what's defect and might bid a little higher.
If I may throw in my advice:
I got the 27" iMac now (i7 Sandy Bridge) and I must say, I adore it.
It's fast as hell if you bump it to 16GB (or 8GB will do for most users, too I guess, I just like to be future proofed and I do lots of editing of photos and videos)
Of cause, to replace an old iMac, that might be a little too much if you just want to get your "old experience" back.
You might be in for the base 21.5" model.
Ask cooky, he has one and he adores it.
I switched him over to the Mac and hence he might have some original input to that.
Either way, even if it's not going to be a Mac, best of luck with selling your baby and getting a new one! Glassed Silver:mac