I have lately found another source of income from all my teaching resources that I have created over the years. All I had to do was update some of the contents to suit present day Maths and Science curriculum. I have started selling on TES. I am getting a steady income of £20 -£30 every 2 or 3 weeks with no effort at all - truly passive income.
There quite a few websites where teachers and tutors can sell their resources. Another popular website is TeachersPayTeachers. The problem I have with this website is it an American website catering for US teachers and schools. I did see UK teachers selling on this website but the listing form is more relevant for the US.
If you are a teacher or tutor then here is an opportunity to sell your teaching resources. Start building your presence on TES or TPT and make extra passive income.
Best of luck