Market Value Question For Graded And Raw

Market value of coins certified or raw why wouldn't we go off of the average of the most recent sales within say 5 years to find market value...?
How often is gray sheet updated?
Where does gray sheet get their values from?
How can a person go off of ebay sold items for any coin that's worth a substantial amount, substantial to me is $800 to 1,000 or more because it seems to me that if you have a coin of that value you would be looking at an auction house and not ebay. I think people are wanting others to use ebay sold items so they can obtain the coins that they want on a cheaper level and then take them to Heritage or add them to their collection. Because I go to ebay sold lists and look at values and then I turn around and see an auction for 20 30 40% more.
If I recall maybe a year ago maybe a little more coins were going for records amounts so just like the stock market why doesn't it the value go up why do you rely on ebay sold listings it doesn't make sense to me an item is worth what somebody will pay for it and going off of an average of sale of documented sales for the past 5 years like PCGS does is the most reliable way they do include some ebay sold listings also...