i think that what something is worth is what someone will pay for it at the time. on tuesday it might be worth $500, because someone wants it. on wednesday only $200 because no one wants it badly enough. the blue book won't tell you that part. if you're trading up, you may do best by using it to get a better deal on the new one.
the other best way to sell is by word of mouth. friends can talk it up for you. the people they know will trust their judgement and not feel like they're buying from a stranger who would take advantage of them.
also, i did pretty well on craig's list, but it took a while. i was glad to get back just a little over 1/2 for a 9" quilter and frame only 1-1/2 years old. it may not seem a lot, but it ended up being the best i could do. so.......
not new is not new. that's why we look for good bargains on ALMOST new machines.