Brand
Isaac Singer and the lawyer Edward Clark founded I. M. Singer & Co. in New York in 1851, and Singer went on to build factories around the world. Most vintage Singers you'll find in America came from Elizabethport, New Jersey, from Clydebank in Scotland (the K models), from St. Johns in Quebec (the J models) or, for the slant-needle 301, 401A and 500A, from Anderson, South Carolina. The serial number tells you which, and roughly when, because Singer gave each factory its own letters. The company spun off its sewing business in 1986, and today Singer sits with Pfaff and Husqvarna Viking under the same owner. Whatever the model, check the bobbin case is there and the feet match the shank, because Singer used several of each.
10 models we cover.

Singer
Straight stitch$470–$1,000 typical ask
198 on eBay

Singer
Zigzag$290–$420 typical ask
51 on eBay
Singer
Zigzag$230–$420 typical ask
35 on eBay
Singer
Zigzag$270–$450 typical ask
30 on eBay
Singer
Straight stitch$290–$450 typical ask
113 on eBay
Singer
Straight stitch$240–$450 typical ask
112 on eBay
Singer
Straight stitch$390–$1,100 typical ask
66 on eBay
Singer
Straight stitch$160–$270 typical ask
121 on eBay
Singer
Treadle$170–$300 typical ask
75 on eBay
Singer
Industrial$600–$950 typical ask
6 on eBay