Copyright can be bought and sold- if you find someone who will sell it to you!! Most artists rely on their copyrights (royalties) to fund them in their old age.
You can't buy somebody's song AFTER the copyright runs out, because... well, that's what copyright running out means. It's then in the public domain and copyright no longer applies.
But you can copyright the arrangement of an out of copyright song. Paul Simon famously did so with 'Scarborough Fair', a traditional song, much to the chagrin of Martin Carthy, from whom he learnt it.