As with any selling platform (e.g. Etsy, Facebook, Craft Fairs etc), if you’re selling goods or services with the intention of making a profit you should check if you qualify as a business. If you do, it’s your own responsibility to register as a business/soul trader. You can be both employed and self-employed at the same time, for example if you work for an employer during the day and run your own business in the evenings.
If you decide to go self-employed, you should register with HMRC (if you’re in the UK). The general advice is that you should register with HMRC as soon as you start working for yourself, but you don’t have to rush to inform them. HMRC state that if you do start working as a sole trader you must register with them by 5 October of your business’ second tax year. So, if you started your business at any time during the 2016/17 tax year (from 6 April 2016 to 5 April 2017) you will need to register with HMRC by 5 October 2017.
In the UK, Disability Living Allowance (DLA), Personal Independence Payment (PIP) and Attendance Allowance are payable whether or not you are working. They are not means tested, so earnings do not affect the amount of your benefit. If you’re in the ESA Support Group you can earn up to £115.50 a week without it affecting your ESA benefit through permitted work, as long as you work for less than 16 hours a week.