For example, if a person creates an application that they open-source under a license that says you can use this code but you cannot sell it, has that person just bound himself to not selling it now as well?
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A license is the owner assigning rights and responsibilities over their intellectual property to a third party. Assigning one set of rights in one license does not preclude them from assigning a different set of rights and responsibilities to another third party.
This is how software products come to be dual-licensed or multi-licensed. Each license grants certain rights but imposes its own restrictions. Thus a software house may opt to release a software product under GPL for free, requiring derivations to be provided with source code, and under a closed source license, for a fee, where derivatives can be supplied without source code.
You retain copyright and as long as you don't sign THAT away you can license your code under different licenses. Watch out though, it may turn out hard to remove a license once you put a license on a work.