Thank you all !!
I understand more now and know that I should learn and exercise with smaller amounts.
I still have these two questions . I would appreciate if someone can answer them.
How can you make money on cex.io if for example 1Eth costs you 800$ but to sell back 1 Eth you only get 700$ back ? Even if you buy low and the price increases by 100$ you would still only break even. Actually it should move up even more up because of the fees.
What exchange sites would you recommend ?
I'm not familiar with cex.io but i don't think there whould be a 100$ spread on the books. In any case, use better exchanges with more volume!
You can check markets, sorted by volume, here:
https://coinmarketcap.com/currencies/ethereum/#marketsSee here 4 screenshots. The first is "buy". The second is "sell". (from BUY/SELL page)
The two last are from the TRADE page.