I WOULD say don't buy a used rope. Just plain ol' don't. I've got too much Seattle Mountaineer training in me, maybe, but that is one item you need to buy new and replace every 5 years, no matter what. Call me over-reacting, but your life depends on that rope.
I've got runners and biners and a harness that are about 9 years old, and I won't use them climbing, even though they've never been in a fall and have been cleaned and stored carefully inside. I won't give them to my grandson, either. Mr. B uses them for fix-it projects (not the harness).