Is zakat obligatory only for the last year when selling real estate, at a value of 2.5%, and if an apartment is sold, is zakat due only on its price, with no zakat due on the remainder?

If your intention oscillates between selling a property and keeping it, then zakat is not obligatory on you. When you sell it, or sell a portion of it, a new lunar year begins for the sale price, and its zakat, at a rate of 2.5%, becomes due after the completion of the year.