They are pursuing debt and evasion of maintenance too.
HMRC rely heavily on handling data, this piece is about tracing those who evade income tax on side income.
3 of the 4 finalists in the last Civil Service data challenge involved HMRC.
One was about child maintenance payments from self employed parents. (These cover approx 17% of parents liable to pay child maintenance but 23% of the backlog)
Partly fraudulent accounting to evade payment and partly the backlogs of processing changes to income
Bear in mind of course that income via eBay could also be hidden from paying both income tax and child maintenance