Sewing machine collectors do not seem to have that "in original condition" thing going on. I would not think average sewing machine buyers would care either. If it looks good, works well and fits in the budget, they will buy it.
However, I've not seen any evidence that there is much money to be made on vintage domestic sewing machines. There are 1,000s if old sewing machines out there. Many are available in thrift stores, estate sales and garage sales at very low prices. If you really get into it, of course you can make some money. I think Miriam has made money at refurbing, but she has a house full of parts machines and is very knowledgeable. She sells parts, too. There aren't many Miriams, though. There is a local guy who posts lots of machines for sale on craigslist.com, so he must be making some money, or he wouldn't continue to do it.
Collectors will usually wait for good deals, unless it is extremely desirable and rare. They usually do their own refurbs, as that is part of collecting.
A warning, though. Once you get it cleaned up and working, you really don't want to let go of it for the price you can get for it. That's how I ended up with so many machines. I gave away my Singer 328 to someone I really like. I kind of regret it, though. I had no use for it, as I can only sew on one machine at at time and I have several more. It was a really clean, well running machine, though.
Another thought - be sure to take your shipping charge into account on the shopgoodwill.com purchases. Shipping can be more than the selling price of the machine. There are also stories of very poorly packed machines bought on that site that arrived damaged.
bkay