There is usually more pipe inside the wall, and most of the time you
can pull it out without loosening the nut. If not give loosening it a
shot, if you are going to mess with this stuff a cheap basin wrench (or
what ever they call that square tube handled spring jaw pipe wrench)
would be a worthwhile investment. If the nut dissolves in your hands
pick up a p-trap at the home improvement store, they aren't expensive,
you can use just what you need & discard the rest or save it for next
time.