If you want to sell a country collection, write to the packet dealers for their price lists. Find the dealer with the lowest prices and divide his price for your country by half for the number of different stamps in the collection. These are two ways to set a base price. All of the following affect this base price. If your collection is full of junk and damaged stamps, don't expect to get even these low amounts.
Condition is important.
It is also important that you have number counts by country before you sell. This saves dealers time, and they may be willing to pay you more. Have really nice items listed with their current catalog values. This saves the buyer time and can increase the amount you get. Please remember, though, that you usually paid more for MNH stuff in the first place plus you had to pay for mounts etc.