I'm not going to play games with  the 1099-K.

I'm going to prepare an accurate return, and put the numbers where they belong (8949/Schedule D for personal items that are sold).  If it doesn't belong on the tax return, it isn't going on the tax return.

If I don't put it on the tax return, I'm going to give the client a pre-written letter so if they receive an IRS notice they can send it to the IRS saying it is not taxable income.  I will also instruct the client that if they receive a letter, they should contact their Congressional representatives to complain that the IRS is wasting everybody's time by sending out such letters.