I have sold a couple machines. Not for a big profit, but to find a better home for it. I figure if someone else will use the machine they deserve it more than I who lets it set. But it also has to be a machine I am willing to part with. I have a few that will never leave my house as long as I have a mind to know what those machines are and what they do. I am attatched to them that much, they are family now!
I have also traded machines with another enthusiast who was willing to work on machines that I had not found the time to work on, and lost interest in them. Another home that wants them is better than left to rust in my opinion.
Follow your heart, and you'll make the best decision for you and the machine. Have I regretted the trades or the selling, no I have not. They were good decisions at the time and for good.