If the owner screwed you over saying that it's not playable online, and does not contain a hardmod than you could contact the owner for a potential refund. But be aware the chances are slim since you already bought it.
If by chance you contact PSN, and say you have been banned becuase of a hardmod again you could try unhardmodding the system, and try asking them if you could be unbanned (This would most likely be ilegal.)
You could also contact PSN to solve the issue for support. Which may be a better route before appealing.
Also unbanning your user could lead to illegal activity since your not following PSN's EULA, Contract, ban activity, etc... And it will only get your other PS Account Banned on your other console.