No, if you are contributing to the site on a less than regular basis (which means you do not regularly prepare and contribute resources to the site), you won’t be self-employed, but you will need to register for self-assessment using form SA1.
If you are spending time regularly on writing or modifying resources in order to make them more saleable, and you are contributing these resources to the site on a regular basis, you may need to register with HMRC as self-employed (using form CWF1). You may also need to pay National Insurance Contributions (NICs).
The information contained in this section will help you think about the tax implications of selling you content on TES Resources. However, we are not tax advisors and recommend that you consult a tax professional to get more advice about your own specific circumstances.