buying and selling rETH again for ETH is only achievable when the staking liquidity pool has adequate ETH in it to manage your trade. This really is Rocket Pool's pool of ETH that arises from two resources:
In Rocket https://dev.to/cleopatramorris_d539d19ea/rocket-pool-the-future-of-decentralized-ethereum-stakingintroduction-1m2b