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The AGPL doesn't prevent competitors selling the software, complying with it is reasonably easy even for a SaaS company.

Then I think it's not what I want.

Its beginning to sound like you don't want an open source license, but a proprietary one instead.

I just want to make sure people can use it for free if they want but I don't want to give the right to sell.

A license that says that couldn't be classed as Open Source or Free Software, but maybe as Ethical Source.