You have to report the income, and pay tax on it, whether or not you get a 1099-K. The delay of the requirement for Ticketmaster to send the 1099-K does not mean that you don't have to report the income. You have $300 of taxable income from the sale.
Furthermore, Ticketmaster already sent the 1099-K, even though they were not required to. So the IRS knows that you received $1,900. But they don't know what your cost was. If you don't report the payment and show the cost, the IRS will assume that the entire $1,900 is taxable, and you will get a bill for the tax on $1,900, plus penalties and interest for not having included it in your tax return.
