Correct me if I'm wrong but isn't it possible to sniff WiFi traffic in the air? If so, what exactly is the point of ARP poisoning? In this scenario, the attacker would already be authenticated on the WPA2-PSK network and can therefore decrypt traffic.
Schroeder is right about the encryption it's uniq for every client but that doesnt mean you can't decrypt it. The problem with wireless is that the content is "in the air" so everybody can capture it. So when somebody connects to an AP and you capture the four-way-handshake including the sesseion specific information and you have the PSK also you can decrypt the traffic of this client.
You can for example do this unsing airdecap-ng from the aircrack suit.
If you have only connected clients you can deauth the one you want to sniff so the client has to reconnect and you can capture the four-way-handshake.