Purchased a 6000 btu unit used from an ad and plugged it in first day and began to work but not it only hums?

Purchased a 6000 btu unit used from an ad and plugged it in first day and began to work but not it only hums? Anyone have an idea what this could be? I am assuming it is the motor locked up.. I plugged it in to a gfi plug outside and it pops the breaker in seconds after hearing a hum... The fan blades will spin freely if i spin them with finger but will not spin when plugged in.. Only thing I hear or see when I plug in ac unit is a hum... Tryed to save a few dollars by buying this "gently used for 3 days ac unit during hurricane" but now it seems something has locked up on it right away.. Can this be replaced or do I not waste my time?