Private person’s purchase for starting a business without an invoice

Dear all, I need to keep accounts from the income and expenses since 30 April 2024. The problem is that I do not have any purchase details, but I do have proof of income. Example: 30/4 bicycle purchased from a private person for a total of 25 euros. There are costs associated with this for which there is no proof (87 euros) and no proof of purchases. The sale was made by a private person, via Payconique. So there is proof of the income. How can I record this expense without supporting documents and without details of the seller?

Thanks in advance, a novice bookkeeper.