I know how you feel. I've never had to sell any Macs, but I did just take apart my first and only custom-built Windows box. It was kind of sad at first, but now I really don't care being a Mac user and all. If it was my PowerBook, however... there
would be tears!
I'm this way with every pig purchase/item I receive actually. For example, I refuse to sell my 3G iPod because it was my first iPod ever, as well as my first Apple product. A friend wanted to buy it off me, but I refused to give it up. He kind of gave me a "haha... okay" kind of reply, but do I care?

The same goes for my PowerBook. Being my first Mac ever, I can't ever see myself giving it up. It's my baby.

An iMac G3 also just joined the Mac family, so that'll be my next project to obsess over. Lots of upgrades to come!
Anyway, yeah, sorry for the loss.

Like others have said, take a picture with it to remember the good times. And plus, you're selling it to a friend... aka someone you should know very well. Visit it on the weekends!