I'm on a 980ti right now and I would really like to go get a better all round PC and upgrate to a 1440 monitor. I will most like get a 3900x with an asus prime x570 pro but I'm stuck on the video card

Would you buy an used 1080ti and if so ... how would you test it ? what's a good price for it ? should I try to get it from a known brand like Asus or Gigabyte ?