I need some help understanding house prices/values. We have talked to realtors who say something like the GV is 370,000 but it will sell for 500,000 or so.

First off, what is the GV and how is it related to the price? Second how do you figure out what the fair value of a house is? Are there services that tell you what houses have sold for? Given that buyers have no representation that I can find how do you figure out what to pay for a house?