Purchases and sale of games on second-hand/marketplace

Hello everyone, I buy and sell games as a secondary occupation. I’m still having some problems with how to get everything okay from an accounting perspective.

If I buy something for 50 euro in cash and sell it for 100 euro via bank transfer to a private person via second-hand:
– 50 euro expense in the cash book?
– 100 euro sale via daily receipt?
– 50 euro expense in the cash book?
– 100 euro sale via daily receipt?
– 100 euro sale via daily receipt?
– 100 euro sale via daily receipt?
– 100 euro sale via daily receipt?
– 100 euro sale via daily receipt?
– 100 euro sale via>
And suppose I buy for 50 euro by bank transfer and sell for 100 euro. Can I then enter the 50 euro as an expense? Because I don’t issue invoices to the person who sells their games to me. In this case, it is not a purchase of a second-hand coffee machine that I want to enter as an expense for my company. It concerns the purchases of trade goods from a private person. How do you enter that?

Thank you!