They aren’t regulated, You can buy them easily online although availability has been sketchy recently.
The one we have at work has a 5 year shelf life and uses off the shelf lithium batteries.
It regularly self tests and shows a green tick if all is good. It just hangs on the wall ready for use.
I give it a visual check each morning as I walk into work.
I couldn’t imagine buying a second hand one. I’m not sure I’d want to rely on one “bought from some bloke in the pub”.