I have nothing to do with this machine, just a FYI.
afew days ago someone ask about one the price was $350, to me that way out of line. The 31-15 I have was $100. it's old state surplus machine, and was from a hospital. (ohs) In super great condition. I also have a 31-20, those do have bigger around bobbins BUT there also narrower. I doubt they hold any more thread, and parts are double of what a 31-15's price.
I see 31-15 often yes often they are 350 , consew version are also around the 350 range .
The same goes on any commercial machine. no different than the singer 15 selling for $800. just wait you'll find a decent price. Even the two singer 16 - 188's I have were $100 each.
This week alone I have HAD to use my 31 and the 16-188 to sew denim basting onto very thich denim quilts. fact I'll be finishing up the last one in a few minutes..
If you don't know a 16-188 is a walking foot machine.
this one has been listed for 3 days, I just guessing it's still not sold
http://roseburg.craigslist.org/hsh/4652314793.html