turn key and no start,starter was replaced with used one...

What tune was the relay humming? I've only known relays that 'clicked',however low voltage (many possible causes)or a locked up engine have been known to make starters hum (usually just a low monotonous tune, no popular songs)..ON a more serious note(no pun intended)..Did you test the starter before installing it ? did you try to turn the motor manually(from flywheel or crank pulley end)?