Several states have programs that buy them at a price high enough to defray installation costs, to encourage adoption of solar. Part of the installer’s job is to do that paperwork for you, so you’d know if you had that option. You can also sell them to other states if you set up with a marketplace, typically SRECTrade. However, the fees and low sale price are prohibitive if you’re not in a state that statutorily buys them for a higher amount

This depends per country. But be aware that if you sell them, the buyer buys the right to generate electricity using fossil fuel, and sell them as if they are green/renewable. If your intent is to accelerate the transition to renewables, you should not sell them.