A Philadelphia county judge recently ruled that unlicensed landlord are not allowed to sue for back rent, though they are not required to forfeit the illegal rent already collected (both sides appealed that decision, so it is not final and may change).
In other words, you have an argument but the caselaw doesn't currently support you. No one knows what the Superior Court will decide, but the county court is not currently on your side.
Sorry if that is not an easy answer — yours is actually not an easy question.
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