Charges by third party websites that list your product for sale and/or connect you to customers can be claimed as expenses in your tax return.
Many platforms will deduct their fees from your total income before they transfer your earnings to you. You should include the total amount of your income earned (that's the figure before any fees are deducted) in your tax return.
If you only report the earnings from the platform after these deductions (rather than your total income), you cannot expense these platform fees again.
Examples of platforms charging fees that can be expensed: Etsy, eBay, UpWork and StockX.