I inherited a brokerage account. I'm looking to possibly sell it for proceeds for a down payment. What tax will I have to pay?

If the account you inherited passed through an estate then all of the securities in the account you received would receive what we refer to as a "step-up in basis".  Further, this assumes that the account was not a retirement fund of any type and consisted of investments made with after-tax dollars. If this is the case, then the estate should provide you with the values as of the date of death. These values then become your cost basis for calculating gains and losses from the time of death on. As an example if the deceased purchase a stock at 20 and sold it at 40, there would've been a $20 capital gain on the stock. The fact that he died with the stock and left it to you means that your cost basis is the $40 a share and you calculate your gain or loss from that cost basis. Whoever the executor for the estate was should be able to provide you with the date of death values and/or contact the attorney or trust company that assisted in the settlement of the estate. Good luck