I find it frustrating to look on Craigslist and find that they not only don't know the model of the sewing machine but they also don't include pictures to see what they are selling. (How crazy is that...to post an ad but to not tell you anything about the machine or post a picture with it. It would be like someone selling a car in the paper and stating..."car for sale" without giving any other information.
I love the vintage machines and enjoy my model 15 treadle. I do have to laugh at the people who are selling their machines on craigslist. They don't know anything about the machines but they have such inflated prices that they make me laugh. Sometimes you'll read on Craigslist about a Singer model 66 (very common model) that they want to sell for ...$5000; or $4000;...on down to $1000. They are usually worth about $30 - $150 depending on their cabinet if they are a red-eye or not.
One guy, who was selling a sewing machine on craigslist (he didn't know the model number) made the comment..."don't offer me any low prices because I know that these machines are going for high prices.
For years, vintage Singers were not a collector's item because they were such a common machine. (You usually found them in someone's barn or garage and could pick them up for $15 - $30. I think sewers / quilters are the ones who can really appreciate the stability and the staying power of the machine.
Yes, I collect vintage Singers...but that is because I love sewing on them. (MY first memories of my Grandmother are of her sewing on her black straight stitch Singer.) :-)
Good luck in finding the model 15 that you are looking for.