If a reward is sold out, it means that players have already purchased all of the available inventory for that reward.  
 
The quantity and type of rewards in the store are based on our partners' availability, so when we have filled all the slots for that day we cannot approve any more purchases. and our reward inventory is subject to change without notice. If you purchase a reward that is limited to "1 per player" you will be unable to purchase that same reward until our current offer for that reward is retired.  Many of our rewards are refreshed everyday — generally around 12pm PST, so do check back regularly!