Is it true if I buy a laptop by December 31, 2020 that I can sell it for half the price? So if the laptop costs 300 euro I only have to pay 150 euro.
No, that's not right.
It would take days to explain
No. A tax deduction doesn't even work like this. If you can prove (!), Then you need the laptop professionally / for further training, etc. Then 50% of the price can normally be set as a private person without any problems. Your taxable income is reduced by this 150 euro, which, depending on the initial situation, brings you back a maximum of 10-20 euro in tax refund (and even then only if you have paid wage tax in that year)