so how do we know that crypto exchngers dont sell us the coins what they dont exisit?
1. Choose reputable exchanges
Ensure that you choose the reputable exchanges, no matter what the exchange. But of course, every exchange has its own reputation. You may seek the most reputable one in your country to get the best one and easily withdrawing your coins into fiat through your bank account. Alternatively, you need also to choose reputable exchanges globally. the exchanges such as Binance, Kucoin, Kraken, Coinbase, OKEx, are some of those that are reputable.
2. Exchanges that list the token/coins exactly
Well, if the coins or tokens are in the top rank and shown in CMC or Coingecko or other crypto trackers, it may be found easily on the reputable big exchanges. But, if it is a new coin that has been listed in certain small exchanges, you must be careful to buy it. For, we do not know whether it is going to be shitcoin or not.
and how do we know those big funds who bought or pretinding to buy or sell some big ammounts of cryptocurrency `?
how do we know they dont lie us ?
There are crypto explorers where we can see the history of the funds. however, if it is about in exchanges, we can believe in the exchange. And as long as we can sell or by what we want and it is fluent, we do not need to think about the funds.
But we may be roe careful if we buy the coin P2P, it may be risky.