first of all when you check the price on some website like coinmarketcap.com you need to also check how often do they update their rates, in case of this site it is 5 minutes.
i am not familiar with the other one you mentioned but check it out yourself.
ETH/BTC on Qryptos 600$
this doesn't make any sense! when you are trading on a ETH/BTC market you are trading between the two pairs called BTC and ETH, there is no $ in the middle.
the $600 you see is a conversion that the website you are checking is doing for you based on average bitcoin price which was updated probably minutes ago, which is why it is different from ETH/USD price which is the real $ value.
ETH/USD is not the same as USD/ETH, right ?
it is the same thing!
Right now on cex.io I can get 1 ETH for 895$, so ETH/USD : 895$ (1 ETH gives me 895$)
but if I sell 1 ETH I only get 773$. This is not USD/ETH : 773$. It is USD/ETH : 0.00129300 (1$ gives me 0.00129300 ETH)
that is because when you sell, you sell to those who are willing to buy from you (the bids) and they are not willing to pay more than $773
and when you buy you are buying from those who are selling (asks) and they are not willing to sell to you lower than $895
instead of buying or selling right away, you can also place an order to go into queue like the rest and wait and hope for your order to be filled.