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I am a qualified accountant in practise. This is what you should do ...
Register for self assessment as a self employed soletrader. You can register here
As you stared to trade around Jun 2017, your first year of trading is tax year 2017-18. I suggest you prepare your first set of accounts to 5 Apr 2018 and annually thereafter to 5 Apr. You should file your tax return for accounts to 5 Apr 2018 no later than 31 Jan 2019 if you decide to file your tax return online.
As far as accounting records go, you should maintain a cash book to record all receipts and payments. If takings are banked then you should keep/retain your bank statement to show all business related receipts.
Where you buy things from boot sales and don’t get a receipt from your supplier, you should make your own receipt detailing items bought and price paid to support your purchases.
As far as first year of reporting goes, in the absence of proper records you should reconstruct your accounts from paypal records and/or any other information you have e.g. diary records.
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.