I actually can't understand why.. and im frustrated because i wanted to buy a coin that was worth 0,73$ on coinmarketcap and on YoBit was 17.77$ !!!!
Afaik Coinmarketcap shows a weighted average. This means that the prices from the exchange with the biggest trading volumes for that coin are taken as the closest to correct.
Oh thanks... but do you suggest me to try... if YoBit it's not a good choice...
Coinmarketcap also shows the exchanges and their volumes. If the exchange has very low volumes you may end up with very big difference between buying and selling prices. My advice is to try to use one of the exchanges in the upper half of the list at coinmarketcap, if there are many choices, or, if the choices are few, just check them all.
Another advice I can give is to use more than one site to check the prices. I use
https://www.worldcoinindex.com because it also shows better the link to the exchange(s) and seems to update its info much more often.