Selling masks to help with the coronavirus crisis?

You know the answer to your question about ethics already. If this offer really was as benevolent as you try to make it sound then you would not be wrestling with your conscience. FB have banned ads for masks.

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Well, I don't see why it's not ethical....it's business. High demand > higher price.
Cost of ads are increasing for chrismas period, airplane ticket is more expensive during summer, a water bottle in eurodisney is 10 times more than the price in a supermarket......it's how you do business.
No one is forcing people to wear mask (and by the way, not sure that those masks are helpful regarding this specific coronavirus).

However, I would see that no ethical if it's selling essential things (foods,...)

This has unethical written all over it, but I don't think you needed me to tell you that.

Honestly, I think that selling an inflated priced disposable mask to a panicked public is a pretty dirty way to make money. I totally get there's a temptation here as you're inevitably going to get sales.

But it goes against pretty much all health advice right now unless people are actively infected, and by selling & targetting ads I think you would adding to the problem, not taking away from it.

If it was being sold for a fair price, perhaps I could get behind the idea.