Scripts to keep the PC out of the house until he buys it are very simple. Check the "Blocked Door" script for an example, It's just an on activate with a messagebox then return. there are several ways to allow entrance after purchase, the easiest would likely be setting a global variable in dialogue when the house is bought and putting the check for that into that blocked door script.
You can handle almost all of it in dialogue...
Here's a sample script:
Begin MyDoorScript;MyDoor is a global variableIf ( OnActivate == 1 ) If ( MyDoor >= 1 ) Activate Else Messagebox "This house is for sale buy it from XXX at XXX) Return endIfendIfEnd
Edit for typO