Before putting your house on the market, attend a few open houses at similar properties in your area. That will give you a firsthand basis for comparing your home with competing properties. It's also a good way to take the pulse of the local market and to spot some of the selling brokers who are active in it.
You can square up a wall before raising it by measuring from opposite corners. Assuming that both the top and bottom plates are equal in length, and that both ends are the same, the wall is square when the diagonal measurements are equal. Adjust the wall as necessary, then keep it square with sheathing or with diagonal braces before raising it.