I agree with Scott.
Further--though I'm not a lawyer, so this isn't legal advice--if your daughter starts charging rent, then she's de facto established a landlord-tenant relationship with the sellers. And that can give the sellers/tenants a whole bundle of rights and protections.
Your daughter's Realtor is the best one to advise her on what to do. But, really, you want the sellers out of there, not continuing to reside there as tenants.
Hope that helps.