I can't speak for all the brands, or all the models, but some hold up very well. I have a Garrett Freedom3 that I have had for 26 years. It does not get used very often, but it still works as good as it did when new. I was sent in ince about 20 years ago when I hunted in the rain a little too long and the speaker quit. Imagine that. If they are cheap enough and you have a bettery or two in your pocket to test tham, I think you will be surprised at how good many of them still are.
Let us know what you find. Be careful and don't pay too much to begin with. Of course there are still people looking for parts such as coils, knobs, meters, etc.
I would personally stay away from the so called "lesser" brands. Just my opinion. A realistic brand detector had a hard time finding a quarter under a newspaper. That was when it was new!
RJGMC
HI BOB!