Okay, I think I got it:
- Ui/Jun/Azusa go to what I guess is supposed to be the Tamako Market market and find the K-ON arcade game
- some girl (Tamako?) was playing it, and shows that she got a Yui card, which Ui wants to buy from her
- while Azusa tries to stop her, Jun notices two other girls play a round of the game (Midori and Kanna?), who somehow get a JUN card (which I'm pretty sure doesn't actually exist yet)
Also, there's now lots more gameplay videos on Youtube if you search for けいおん!放課後リズムタイム
thanks for your attempt to translate it.