Like I said, your implementation would depend on context, but to adapt my example, I'd probably put a function on the Property class to determine which Contract class to use - something like:

  contract = property.create_contract(seller, buyer, agent)
  contract.sign()

However, I think from what you're saying that your sell_property() function would need awareness of all property types, so adding a property type not only requires changes to the part of the code which contains the property data structure, but also to the sell_property() function - and any other function throughout your code base which uses properties. That is of course possible, but OO does make it easier to find where to implement logic specific to different property types.