Should one buy a used Brooks saddle that is already broken in?
I've purchased a couple of used leather saddles. The ones that were barely used or just broken in were fine, but there was one well-used one that was the exception. Its sit bone area was visibly lower on one side than the other, enough that it was the likely cause of an SI joint dysfunction for me.

I'd say if it looks relatively new and close to the original shape, you're probably fine. If it looks worn or stretched and especially if it looks uneven, it might be best to pass.