I don't have a screenshot or something to prove it. It was early 2017 and Poloniex was doing what I did talk about. Now I am sure that Binance's selling non-exist coins to it's customers. Okay, I can't prove it but it deserves being a theory. There are too many people try to earn easy money on the Exchanges. Therefore, it will never be more than a theory until the regulation or greedy people decide to withdraw all their assets their own wallets. I'm gonna wait for the first one.
Well, it's definitely possible. Just like you said if everyone withdraws we'll see whether those exchanges are doing some dirty things or not. To be honest I'm not surprised if this happens on low volume/low-quality exchange where you can't even withdraw $50 from your balance.
This is why Paypal selling bitcoin is a very risky service, at least for me. Like others mentioned above, the chance of getting scammed is high, even if the company is reputable (since we don't really know whether they have any bitcoin or not). If you do believe an exchange is doing something shady, feel free to leave them and make a thread about it. Pretty sure it will help people as long as you get enough proofs about the order book and stuff like that.
I've already left the Binance and removed my $VET and $WAVES to my own wallet. I'll be watching the market and I'll make a thread about it with more details. Exchanges will have to stop speculate and remove fake resistances. Cryptocurrencies can't drop like %90. Infinity high values in their nature.