I had a magnet do that to me once. Turned out there was water that got into the sealed magnet and started to oxidize the metal. The magnet was put together in layers (like a tree trunk with the shaft in the center) so it caused the outer layer of magnet to push the sealant coating into the impeller pump housing.
The striations seemed to be caused by the parts of the magnet swelling at different rates.
How did it happen? I assume something harder than the sealant got into the housing and punctured the impeller allowing the water to seep in. It may have been a manufacturing defect as well - who knows?
Anyway, inspect the inside of the magnet housing (pump side) for any damage. If it is good, then get a new impeller and you are good to go. If not, I would not use the pump as any damage to the pump may lead to stray voltage being introduced into your tank.
Either way I would let the seller know about the damage.