I would guess:

• Moon landing was faked
• Anti-vax stuff.
• The entire scientific community and goverments are making global warming up and hiding evidence to the contrary. Somehow, for some reason.

Popular in media but probably not many actually believe:

• Space aliens are here and goverments somehow are able to hide that. Good luck with that I say.
• The eart is flat and all of academia is teaching the wrong thing. For no reason.

Try selling diamonds jewelary on the second hand market. You’ll get the gold value.