Fact.  It's a legal requirement for ebay to provide sales data for individuals to HMRC when one of the two thresholds is breached.

Fact.  It's a legal requirement for ebay to provide the individuals NI number with that data.

Fact.  Your advice (and what you yourself are doing with your accounts, as stated by you), is specifically aimed at preventing ebay fulfilling its legal requirements.

Fact.  Posting that advice is against the community board rules.