I don't know if they any kind of warranty on used guns. I've dealt with their customer service and they've helped me, with fairly speedy results, by sending parts free of charge. I've no first hand experience with returning a firearm to them.
In regards to the slide locking back:
Check that the slide stop lever spring is not bent and is in the proper position when the stop is installed.
There is also the possibility that rounds are hitting the stop as they are feeding. They can do this becuase of bullet shape/length, by moving slightly forward before feeding, or an over-sized stop. Try working the slide by hand while watching the lever for movement.