You looking for 12.5 and earlier or 12.6 and later? You looking for Standard or HD?
Jeffro's comments in this thread are very helpful.
https://duc.avid.com/showthread.php?t=414187
Pro Tools 12 used license costs should be low cost because of difficulty obtaining a way to update the software. If buying online there are lots of scams out there (so maybe asking on DUC like you are is a good idea).