What is the function of 便 here? And what does the sentence mean?
不猜便罢, 若猜便猜它个八九不离十


Yes, 便 here means 就. The whole sentence means 要么就不猜，要是猜的话就会猜个八九不离十. In English, it could be like: Either they don't guess it, or they do - and they'll probably get it right.