General attitude or game ettiquette and buying props

[Richard]

I scanned the rules and don't see it even mentioned - which gives me the impression that buying props is part of the game and as long as the property is for sale it's available to buy without hard feelings.

Exactly. There's no such rule about not buying people's properties. After you buy a property, it remains yours for 10 minutes minimum. After that, it can be bought by other people. That's how the system works.

Of course, people don't like it when you buy their properties(gives them less profit) but they are for sale again and you can't stop that.

So, there's nor a written nor an unwritten rule about it. People just don't like it but can't make you in any way but killing / hunting you down.