The economics of continually selling a product for under its market value?
1st of all, you should distinguish cases from the Nike case and others, if any:

Since Nike sells only a tiny portion of its shoes "irrationally" it cannot be seen as market value - equilibrium price, but rather as marketing expenses

Think that simply generating a rare nike products market adds value to the brand and motivates consumers to prefer nike

Regarding any other case: they can either be "dumping" - that is selling at a loss in order to throw your competitors, selling above your *average * cost, meaning making  a profit, maybe fightinv for market share, or being cross funded by different activity, to hurt competitors in the subject market