All very true. I belong to another forum that makes it very simple. It's a rule there: "We don"t do appraisals period. ".
The reasoning is that there are too many variables and someone is bound to get their feelings hurt.
The best thing I know to find out a price for something (not just sewing machines) is to watch your local ads for a while to get an idea of asking prices (definitely not the same as actual selling prices) and check closed Ebay listings for actual selling prices. After a little while you'll get a feel for relative prices in your area.
Rodney