They don't. It's automatically calculated through special algorithms that really have no bearing on actual "fair market value". Please don't pay any attention to the values you see here, or on other sites like Zillow or Realtor.com.
Whether you are buying or selling, the ONLY way to find out a home's value is to have a REALTOR do a FREE comparative market analysis (CMA).
Buying a house off the court house steps is risky business. You need to check the title before bidding to know that you will get clear title. You will still be responsible for unpaid county taxes. Also you have no right to go in the home before the foreclosure unless the owner invites you in. You could be getting a home that needs lots of repairs you have not budgeted for.... more
Rodney is right... your bank will assign the appraiser, you do not choose them. If you are considering buying the REO why not have a Realtor do a Market Analysis for you as-is or as-completed ( if you are flipping it ).... more
You need to check with the county planning office to see that it has all the required permits. The last think you want to do is by a house you cannot get a CO for. Get a good home inspector. I know one that uses a inferred camera to scan the walls and ceilings.... more