You'd need to get a receipt somehow from that individual who sold you the collection.

The way I understand US tax law: Anything you spend as an expense to conduct business is a deductible cost from your business income. And that would apply to singles or boosters.

Once you branch beyond "Hobby" status, and are reporting income as a business, your entire operation is part of that business, so any expenses you have are deducted from your annual income before you pay taxes. There would be no benefit to separate the singles portion as part of your hobby. Being a business has some positives, For example all your costs are deductible, even if you are losing on the year, you'd get a tax credit on the next year, or a refund if you've pre-paid any taxes.

My main concern is how to report the following: I buy sealed items with a vendor and sell those items to customers. Normal business. A certain percentage I open myself (boosters -> singles) and I sell the singles.
Do I sell those boosters to myself and consider the selling of singles as hobby?