If you want to take the illegal route than you could unban yourself. But that may go agasint PSN's EULA, and contract.
I sadly do not know but there are various methods to unban yourself if your willing to take the time, and learn how. I never done it myself but I'm avoiding it to prevent dealing with issues with PSN's Contract, and the EULA.
i also heard that if you could grab another ps3 id preferentially a broken one or a console that barely is used online, you could swap the ID and unban your ps3
i remember there was a tutorial here at gbatemp..
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.
staff is just utter disgrace and waste of human space, just like this site is a waste of server resources, i wonder how much crypto i can mine by reformatting teh server for gbatemp and just running crypto miners on it...