Buying vintage, older or used thread can be a deal for the pocketbook, but may lead to headaches down the road. (Please note that I am a huge fan of buying things secondhand.) Some of these threads are useable and great, and some may have become too brittle to use. Here’s a quick check for useability.
Unwind a length of thread and try to pull it apart with as much strength as you can. If it holds up, great! If it breaks, then it will probably break while you are trying to close a ring or hide ends. Don’t use that thread.