I also have a similar situation, but I would like to avoid ebay if possible. Any other suggestions? I plan on posting some fliers and telling all the people I know, but what if that doesn't work?
I think selling stuff on ebay is pretty safe. Of course, you want a buyer to pay before you ship anything, and putting enough 'small print' in your ad (computer sold as is, all sales final...) will protect you against a lot of potential hassles...
Also, some Mac stores will let you trade in your old computer when you buy a new one...