Effective (experienced) day traders actually dampen volatility. It's the panic sellers and buyers (amateur traders) that create volatility.
1) What exchange do you use now? Coinbase makes you withdraw to bank account when you sell right? Does Bitstamp allow you to retain a USD balance, so you can buy back in?
Yes, Bitstamp allows you maintain a USD balance.
2) Is there some very basic math you always use, because of fees? Like the increase in BTC price must at least be above X % ... so the fee taken out by Bitstamp is covered?
Yes, it shouldn't be that hard to derive the proper formula if you think about it.
3) Do they charge a fee to buy and a fee to sell? Whats that fee?
Yes, they do. You should go to their webpage and check it out for yourself.