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!