Long as you're sure the new owner registered the car, then you're likely fine. Most often the issue arises when one leaves their old plate on. Fortunately, you took it off, which substantially reduces the risk.

Personally, I'd suggest mailing it back in a USPS Priority envelope. That way you have tracking. Then sometime afterwards PennDOT will mail a letter indicating the registration has been cancelled. Vast majority of people don't do this and have no problems, but for several dollars, it's worth the peace of mind.