njs is a japanese organization that regulate japanese track racing (keirin).
njs frame brands are
1. bridgestone
2. panasonic
3. cherubim
4. makino
5. kalavinka
6. levant
7. nagasawa
8. nanbei
9. shimazaki
10. rap
11. eimei
12. watanabe
13. umezawa
14. samson
15. level
16. yanagisawa
17. gunwell
18. georama
19. civras
20. vogue
21. iribe
22. kiyo miyazawa
23. peloton
24. baramon
25. osca
26. ohtaki
27. stratos
28. giro
29. bomber
30. pro cyon
31. presto
32. emme idea/m idea

some of these brands did make other, lower level frames eg. bridgestone/panasonic, so i wouldnt jump on either of these brands if you see them cheaply.
although i highly doubt you will see any njs frames at garage sales in the US, as they were never distributed there, what you want to look for to know if it is njs is the logo stamp.

which looks like this.