Well, you need to know what you want. Singer 66, 99, 15-91, 201, 221 Featherweight and 301 are straight stitich. 401, 501 rocketeer have build in stitches, Singer 237 has zig zag and this is not a complete list of old Singers out there..I stay away from Touch and Sew (Touch and throw) due to plastic gears but your milage may vary.
So I decide what I want (I have all of the above except 221) and then take thread and bobbin and scrap and make it sew. I can fix tension issues and I can clean and oil myself so there is no take to service man. If it don't sew, don't go. That happened to a FW I looked at once.
You always take a risk. I rarely have been bitten on Singers. I love the old stuff. I quilt, piece and machine quilt and fmq. Read more about it? Yahoo has a vintage singers group that is VERY extensive in its files, so read before you ???. Oh and try to pay $50 or less, these machines were mass produced. I have paid more but never more than $100 (FW excluded). The bit about straight stitch machines worth a mint is only in the eye of the beholder. CL sellers should keep their overpriced, stuck, filthy, beat up, abused singers. For the right price and the right machine, I will do spa treatment. Good luck....