Open source == no protection. You rely on the morals of your customers. At any time somebody bigger than you can take your free source, modify it, make it better because they have already a base and sell it as closed proprietary software and you can't do anything about it.
This is not true for all open source licenses. Copyleft licenses such as the GPL and its variants prevent this.
Someone can distribute your software instead of you and thereby lock you out of any profits there, especially if they undercut your price (in the limit case they can distribute your paid software for free), but certain open source licenses prevent retroactive locking as proprietary software.
