Only way I could make a little money on the old machines was selling Singer 15-91', 201-2, and the older 15 Tiffany, 66 Redeye, 27 sphinx heads with decals in very nice shape on ebay and ship. Surprisingly the 301 and 401 didn't do very well. Never could find the foreign machines (necchi, pfaff, Bernina...) with excellent reputations at a reasonable price around here to try out the market. Local CL market is pretty bad unless I want to donate. This is just my experience and may differ elsewhere. I collect Davis' which seem to have very little value which is good for buying. Seems the money is in servicing rather than selling the old machines but the "horse trading" is great fun and rather inexpensive.
Happy buying, trading, and selling
Jon