If you are reporting the income on schedule C (as a business) the storage cost is deductible, since it is only used for business inventory. Only the current year cost is deductible on your 2020 return. It is not part of cost of goods sold. You enter it, on Schedule C, as rent or other expenses. If you reported a Schedule C in 2019 (or previous years), you could amend 2019 to add the 2019 storage cost.
If you are only repotting the income as hobby income, or as capital gain on the sale of collectibles, the storage cost is not deductible.
