If a landlord sells the property, can they have people come over on a regular basis to inspect it?

Your landlord can send people to inspect the property but she must give you notice.  Each state has different notice requirements before someone can enter your apartment/home you rent.

So, in your state, that would be up to the lease agreement.  Most states require reasonable notice with no notice required in emergency situations.  Pennsylvania doesn't seem to specifically spell out the right to enter but again it is an issue of reasonableness and don't forget you still do have a right to privacy.