In this case, I don't consider signContract part of the seller's or buyer's role in that signing is more closely aligned with a contract than a person. That is, a person does a lot more things - views house, negotiates, etc. If you put all these methods on person you could end up with hundreds of methods on person and violate SRP. Thus, it seems to make sense to me to have

contract.acceptSignature(seller); contract.acceptSignature(buyer);