How do you buy land usually?

You go to a broker, who remembers all the plots/flats available in that society and the ask prices . He may optionally ask you to bid a price that you are willing to pay

How do you sell land usually?

You go to a broker, who checks all the bids for the houses in the society. You optionally give him an ask for your house.

How does the final transaction happen?

If the orderbook ever overlaps, the transaction can be made (unless one party backs out ofcourse)

New orderbook