There are specialist sites that sell Aga's ,or the shops that sell them also buy back unwanted ones. Contact one and they will come and make an offer (they need to see it ) and pay you cash if you accept. Always use a specialist shop as they have the equipment and knowledge to remove it safely,quickly and cleanly. But for that cost the Aga is going to be running the central heating and hot water system as well. So you will need that looking at and costed.