When you create a node, the 10 STRNGR is a contribution, not a deposit. As stated on the create node page:
A contribution of 10 STRNGR to the StrongBlock community is required to create a node and participate in rewards. Your 10 STRNGR will be distributed as follows:
10% future use
90% into node rewards pool
The 10 STRNGR contributions cannot be returned, it is used to further the protocol. Selling a node is also not possible, a node is tied to your wallet.