I sold $22k in sports card (700 transactions) and got a 1099-K from eBay. Some for a gain and some for a loss. Do I need to enter each sale individually under Inv/Saving?

NO. If it is a business it gets reported on schedule C. if it is a hobby (ot a business) see this link 

https://www.thetaxadviser.com/issues/2019/nov/taxation-collectibles.html 

The question you raise is not one that has one solution.  You may need to discuss your tax options with a tax professional.  However, here are some things to consider.

Were these sports cards capital assets?  In other words, were you buying and selling them for investment purposes.  If so, then you could report them the same way you would report the purchase and sales of stock, namely on Form 8949, Sales and Other Dispositions of Capital Assets. Reporting this way, you will have to determine whether the sale was long-term (held for more than one year) or short-term (held for one year or less).  Any capital gains can be offset with capital losses, plus $3,000 of ordinary income.  

Because it is unlikely that your basis in the sports cards was reported to the IRS, you will not be able to enter a summary of your purchases/sales on Schedule D.  Thus, you will need to report each individual purchase and sale on Form 8949.  

In TurboTax, a 1099-K, is entered in the Other Common Income section under Wages & Income.  1099-Ks generally just report gross proceeds, so any expenses you had in connection with buying and selling sports cards will not be included.  You will have to calculate your expenses separately.  

Lastly, was this a business your just a hobby?  Unfortunately for people who earn income from hobbies, the Tax Cut and Jobs Act (which went into effect in 2018) eliminates the itemized deduction for hobby expenses, along with all other miscellaneous itemized deductions. The prohibition on deducting these expenses continues through 2025. This means that taxpayers will not be able to deduct any expenses from hobbies through at least 2025, but they still must report and pay tax on any income they earn from a hobby.  The deduction is scheduled to return in 2026.  Below is a link to IRS information about hobbies but also information about what constitutes a business.  

What Taxpayers Need to Know About Paying Taxes on Hobby Activities

@acoiner456