Is commission-only work legal?
Yes, where the relationship is genuinely self-employed. National Minimum Wage rules apply to employees and workers, not to self-employed contractors selling on their own account. That is why the independence has to be real: you choose your hours, your campaigns and your methods, and nobody at RingoMarket directs your working day.
Where the line sits
An arrangement stops being self-employment when someone else controls how, when and where you work. Assigned shifts, a supervisor directing your calls, or a requirement to work exclusively for one brand would all make you a worker; and workers are entitled to at least the minimum wage. None of those exist on this platform, by design rather than by accident.
What you take on instead
You register as self-employed in your own country, invoice for what you sell, and file your own return. In the UK that means telling HMRC you are trading and filing one Self Assessment a year; below £1,000 of self-employed income you need not declare it at all.
This is general information about how the arrangement works, not tax or legal advice for your circumstances. Thresholds change; check GOV.UK or an accountant before you file.