That's how centralized exchangers work. It's about building trust. Just like how you would trust Binance by depositing some dollars or Euros with them before you buy the Bitcoins. Bestchange is a monitor that compares the rates. You can also be able to see some more information about the exchangers like the past reviews, age of the exchanger, the reserves and how long the exchanger has been listed on bestchange among other things.

I would suggest you make full use of the reviews feature before you get to use any exchanger and also thoroughly read their terms and conditions.