1. How do I sell my bitcoins without any noticeing my wallet and who I am? Should I always use a vpn when sending a transaction?
it depends on the type of wallet you are using!
if it is a full node then using VPNs and TOR are good options for increasing your privacy but if it is an SPV wallet, a web wallet or worse of all a custodial wallet then your privacy is reduced in order of the types i listed here. hiding your IP doesn't do you much.
2. Is there criminals that searching in blockexplorer (or in another way) to find a wallet like mine and then searches up those ppls in the real world and threats them to their life?
they have to be able to link the "coins" to the person. as long as you are not advertising your identity online and not linking it to your address you could be fine.
3. How do I best sell / splits up my wallet
sell on exchanges, and if you have large amounts split it among multiple addresses (like 10
BTC per address).
if you decided to sell face to face then make sure to do it in a public place and bring someone with you as "bodyguard"!
4. Can I trust the exchanges? They have my personal information.
not so much. they are hacked all the time and some of them are shady enough to sell your information online! specially the big altcoin exchanges.