Does the seller side has to have the bitcoins when the contract is made?
No. That's the whole idea behind "regulated" bitcoin markets. You don't need to have a single satoshi to sell thousands of bitcoins and suppress price.
Bankers think that bitcoin ETFs will solve bitcoin supply "problem".
When the contract expires seller has to anyway have to get the coins from somewhere to be able to sell them to the buyer?
Nope. Actual delivery of bitcoins is forbidden. Just like in gold futures and ETFs. At the expiry date profit/loss is cleared through the seller's margin deposit which is in dollars. If the seller is systemically important bank and they did the wrong bet Fed will print new dollars and will give them to pay for their loss.
The buyer side was there to buy some bitcoins. If he will not have them, what will he have? The dollars?
The contract said for example buyer will buy three bitcoins for price 3500$ per one coin which is total 10500$. But the day price is 4500$ per bitcoin which is total 13500$. So the buyer will have 3000$ for bank account?