Expenses with left over stock

Hi, I'm looking for some advice as I cannot find much online. I've started to buy and resell trading cards as a little side hustle hobby and my sales will go over the £1000 threshold for this tax year. What is unclear to me is regarding what can be put down as an expense. For example, if I were to buy around 200 cards to sell during this tax year, but let's say by the end of the tax year I only manage to sell 150 of them, can I still put down the initial cost of purchasing those 200 cards as an expense even though I only sell 150 of them? Or, can I only put down the expense of the actual cost of goods SOLD and therefore can I only put down the cost of the 150 items sold in that tax year (I hope this isn't the case as that will mean complex calculations of apprortioning out cost of decks etc, which kind of defeats the purpose of me doing this as the hobby no longer becomes enjoyable).

Hi,
You can use the cost of the actual items bought in the tax year and then your turnover is the proceeds from those that have sold.
Thank you.