The first rule is... there are no rules (unless you have a purchase agreement signed) - just protect yourself as a buyer. The seller can do whatever he wants given you have nothing in writing, which seems to be the #1 justification I've seen sellers use to pull moves that could be considered "rude" or "shady". Even then, having writing/contracts isn't the end all be all.
Super cliche but don't be afraid to walk away from a deal if you don't like who/what you're dealing with
