Retirement tax questions

You report it as investment income, under "Stocks, Cryptocurrency, Mutual Funds, Bonds, Other." When it asks for the investment type, click "Other (land, second homes, personal items)."

The assessed value for real estate tax doesn't matter, and you cannot deduct the cost of maintenance. Your capital gain will be the amount you sold it for minus what you paid for it. It will be a long-term capital gain.