Is it safe to use a used Bluetooth keyboard?

If you're just an everyday person without any special security concerns, it's likely okay for home use. It’s actually pretty difficult to modify a keyboard to carry malware without the original owner having a decent level of skill.

While it's technically possible for a Bluetooth keyboard to have malicious firmware installed, that's quite rare unless you're a high-value target. Most keyloggers are more often found in separate hardware or malware on your computer. If you really feel at risk, tools like Wireshark can help monitor any weird traffic, but for most people, your standard security software should do the trick.

You could use something like Dettol wipes to give it a clean, but honestly, I'm not sure what you're worried about. Bluetooth devices have a pretty limited range, so unless someone is directly targeting you, it should be fine.

Honestly, it's very unlikely to be unsafe. You should be good to go!