This max supply of 21 million is an arbitrary number and says absolutely nothing.

It's the amount of units in which you can divide this supply, that counts!

Following the current specs, there would be a max supply of 2.1 quadrillion units (satoshi's) available. That's a LOT. Much more than there are cents in any other currency in the world (although they are unlimited because governments and banks can create them out of thin air as much as they desire - which is the cause of an entirely different plethora of problems, but that's an entirely different discussion).

Furthermore, should the need arise (which won't happen anytime soon), this could be easily adjusted and we could increase the precision to 16 decimals and even beyond that. This would allow for a worldwide supply of no less than 210 sextillion units. Every single person on earth could have more of these units than there are cents of any other currency in existence.

So, no, this is not a problem

You would have to send whatever currency the seller wanted... As an fyi, not too many people are keen on doing paypal for btc since there is a massive risk of charge back. That being said, there are always people that are willing to take the risk