Self-Employed business registration selling used items in Europe

Hello everyone,

I sell used items online which I buy without invoice nor receipt so I am looking for a tax form in a country which forfeits the cost of business, otherwise I will have high income tax and no deductible costs.

I know Italy has regime forfettario but social security tax is very high.

Do you know any other country in Europe that would be suitable for my type of business?

Thank you!