1. In venmo account you press "Charge" , enter the amount and my mail .
2. I acknowledge your request and you'll get your money .
3. I can not get them back then .
You would be able to claim your account was hacked, or you could actually have hacked. If you used a bank account or credit card to fund your account then you could claim the transaction was unauthorized, or the transaction could actually be unauthorized.
Anyone who sells Bitcoin to you is going to need to trust you to not attempt to chargeback the transaction. In order for it to be appropriate to give you this trust then you will need to have an extensive trading history that can be verified; a trading history that you do not appear to have.
My suggestion would be that you use cash to buy Bitcoin as you will end up paying a much better rate.