The stake is not used to pay, nodes are free (as much as you set it up yourself). Then to create blocks and thus get part of the reward, you must add the node to a registry. There is a queue to enter the registry, as only a few nodes are allowed per time. The queue can be escalated locking funds. Once you want to recover your funds, you are out of the registry (and thus stop earning rewards). Quite simple. The coins remain with you. The only thing that is for sale, is to have a node account already in the Genesis block, so that at the start of the blockhain you can immediately earn coins.