。
Eggs are sold at the supermarket.
Literal
Egg [topic-は] supermarket [at-で] are-being-sold.
The passive 売られる + the ~ている auxiliary = 'are being sold / are sold [as an ongoing state].' The で marks the location where the selling takes place. スーパー is a shortening of スーパーマーケット, the standard everyday Japanese term for a supermarket.