That's free market capitalism for you. Its a matter of supply and demand. Naturally, when demand goes up, prices typically go up as well. Its why rare sports cards often sell for tens of thousands, and sometimes millions of dollars, because there's enough demand to drive the price that high.
You say its price gouging, I say retailers have to make a profit. In the case of those masks, the higher demand from consumers could also be driving up the prices from the suppliers, forcing retailers to raise their prices as well. Likewise, manufacturers will typically raise prices when production cannot meet demand.
As for this being a public health concern, that's not up to ebay nor us to decide. If political leaders see it as a threat, they may do something about it.
Either way, our entire world economy works on the premise of price gouging. If you're not making a profit, you're not in business. I would agree a lot of prices seem unfair and unreasonable, such as with the enormously inflated costs of healthcare.
