1099-MISC box 7 check mark $5,000 sales

My opinion - you are not responsible for your client's poor bookkeeping and should not band-aid a tax form to help them out.  Take the $5,000 into a Schedule C and based upon that prepare an extension.  If they complain, tell them it's their responsibility to give you proper information as you could be charged preparer penalties for an incorrect return.  If they wish to take the return to another preparer, fine.  If  lousy bookkeeping this year, it'll be lousy in the future and you may not want that type of client, and that risk.

"Do I add $5,000 to the top line on sch C?"

This $5k is not additional but is informational and would already be part of the financial data from the business operation.

What this means is that there are Purchases for that amount, for your taxpayer's business to sell. That is not Revenue, but Expenditure, typically as inventory cost. Your client has to report to the best of her ability her entire business operation. Whether these items are also sold or not will be reflected in the data you get, which also means you ask for starting and ending inventory information as well as purchase amounts.

If none of that is already sold, it is inventory asset. Not income and not expense.