There's no rule about it, so I'd say it's fair play.

However, it's a social game so if you keep 'crying wolf' then you might find your game-mates less likely to trade with you!

Is pretending to want to trade before playing a monopoly card objectionable?
If $f: \mathbb Q\to \mathbb Q$ is a homomorphism, prove that $f(x)=0$ for all $x\in\mathbb Q$ or $f(x)=x$ for all $x$ in $\mathbb Q$.

I'm wondering if you can help me with this one? 
