I'm not a fan of selling computers or any electronics. Usually I keep my computers and use them as backup machines. I haven't had that many computers to really have a plan what I'm going to do for them but my old Acer is currently somewhere in the house used by other family members. Currently I have 24" iMac and 13" MBA which both are used daily. iMac's main duty is to serve as a media center and server but I still use it for normal tasks daily.
Since Macs have higher resale values, I've been thinking about selling them in the future. Right now, I would be interested about selling my iMac and building a Hackintosh as there are certain features that I would like to have (internal HDs, better performance, Blu-Ray, upgradeability...) but I think I'll pass. Maybe later.