the only think that worries me a little and making me think of selling some of my sites is the boom in the market like it happend few years ago..of course we cant compare those days with now days..but still life is a cycle..
A lot comes down to time. If you have a few sites, and they begin to suffer due to not having time to work on them, then it might be better to let one go, so that you can concentrate your efforts. This way you get money for a site, and you should be able to make extra money from your remaining sites.
I usually consider selling a site after seeing its earnings drop below average for 7 consecutive days.
well are you going to make enough right up front to invest intelligently in other places?? and how much are you being offered on the site?? how much are you making from the site?? I'd only sell it if that is a real simple decision to make and you are ready to stop dealing with the site itself.