Before selling stamps, or your entire collection, you first need to do some research and work, in order to come up with a realistic value for them.
First, examine all your stamps for faults. Any stamps that have faults, such as being stuck to the album page, thin spots, damaged perforations, stains or discoloration, etc., should not be counted in your evaluation. It may be wise just to remove such items completely. If a potential buyer looks through your album and notices a lot of damaged material in it, the offer you receive may be drastically lower than you expect.
When selling stamps, you want the appearance and organization of your collection to be as attractive as possible. Buyers often do not even look at the common material. They examine a "few" of the higher-catalog value and classic stamps, then make a judgment on the quality of the collection from that. If the material isn't attractive to the buyer, the potential buyer may lose interest in making any kind of offer on the collection.
Now, it's time for some real work! In selling stamps, you will need to calculate the approximate WHOLESALE value of the collection. Just about all libraries have a current set of the Scott Standard Postage Stamp Catalogs in their reference section. Larger libraries may even have older editions that you can check-out. You will need to take your collection album, a note pad, and a pen with you.
Per the Postage Stamp Values page, in this section, and the Stamp Condition section of this website, you will need to do the following. Count all U.S. mint stamps after 1935 and all the foreign mint stamps after about 1955 at their FACE VALUE, converted to U.S. Dollars. Do not count any stamps with catalog values of $0.25 or less, as these are what's referred to as "penny stamps", as stated in the Postage Stamp Values page.
Now, with the older stamps that are in GOOD CONDITION, calculate them using the Scott Catalog values. You don't need to list each stamp individually ... doing an approximate catalog value for each album page is usually sufficient. When you are done, add the values up and calculate 10% of the total Scott catalog value.
Add your 10% value for the older stamps to the face-value total of the modern mint stamps. This will give you the APPROXIMATE WHOLESALE VALUE OF YOUR COLLECTION.
Now, you have to find someone to sell the collection to. The next couple pages in this section should help you with that.