A seller should never open a dispute (except for unpaid items which should alwys be pursued.) If you lose it goes on your record.
Tell him to sue.
If it goes to Small Claims Court you will not need a lawyer, although betting some advice from a legal aid clinic, like those attached to universities with law schools may not hurt.
More likely he will go away.
You can deal with the harrassment by asking your email and phone providores to block his attempts to contact you.
Since he inspected and paid cash, there is not much he can do.