When does selling on TCGPlayer become a business and not a hobby in terms of Taxes?

Hi all,

This year I started selling some cards on TCGPlayer after a couple of years buying and collecting cards. I received a 1099-K in the mail for my sales, totaling just over $1,000. This whole time, throughout my time opening cards, I always just assumed it was a hobby and never put much thought into it, but now with this form, it feels like things changed, and I could use some help navigating this.

• Is my buying and selling cards now considered a business in the eyes of the IRS?
• Does this mean I have to fill out a Schedule C for this too?
• If this is a business, what counts as expenses? Shipping? Stamps? Top-loaders? Packs?
• If I spent say $2k on buying packs and boxes this year, and sold $1k worth of cards, am I in the hole $1k?

Appreciate any and all help!