buying and selling rETH back for ETH is simply feasible once the staking liquidity pool has ample ETH in it to take care of your trade. That is Rocket Pool's pool of ETH that comes from two resources: to be a staker https://dev.to/cleopatramorris_d539d19ea/rocket-pool-the-future-of-decentralized-ethereum-stakingintroduction-1m2b