And beyond legality, kids self-administering HRT with neither parental nor medical supervision is a truly scary thought. It is certain to leave most with significant health issues, that no amount of good will will help them solve.
Ideally yes you would want a GP to prescribe you HRT. But if in the place where you currently live this is not possible without years of gatekeeping, or parents have veto power, gray-market solutions are the next best possible alternative.
I fully support the right of children to receive puberty blockers to help them achieve a smooth transition when they are old enough to fully understand the consequences. I bemoan states and parents that seek to harm trans children by refusing them access to this type of treatment, and others. But slef-medicating as a child is in absolutely no way an acceptable replacement. Being a non-passing trans adult, as bad as it is, is likely to be much much better than the harms you expose yourself to by accidentally overdosing on hormones as a child.