Fine-tuning/calibrating DS18b20 thermometer via 1-wire
Unfortunately, fake DS18B20 devices are well known to be (a) inaccurate, and (b) have varying readings even in a stable environment. The chances of your devices from eBay being fake, is approximately 100%.
This graph, taken from this answer on the earlier question: &quot;Which one of these DS18B20 temperature sensors is fake?&quot; shows the problem. The one genuine device is obviously the stable, top red line. The other 4 unstable and inaccurate readings are from the fake devices.

Answers to that earlier question have several useful links, including this GitHub repo:
https://github.com/cpetrich/counterfeit_DS18B20
which has two Arduino sketches that can help to identify fakes.
Useful search terms include - DS18B20 counterfeit accuracy