People will buy anything

That’s not true, not anymore. Especially now that people have so many options, so many ways to research what’s being offered, so many channels to buy from. If you’ve tried different audiences, if you’ve tried different creatives -in your online campaigns-, and people still are not buying from you, it might be a good time to revisit what you are offering. If you are selling an online course for example, before spending money on ads, try to publish it in an online marketplace, see if there is any organic response first.