I've mentioned this in another thread, but if you're a beginner collector and merely want to review Scott catalog prices, get hold of a copy of the Postal Service Guide to US Stamps. They are available at many larger post offices, or you can order them online at this link:
https://store.usps.com/store/browse...desYearbooksAt $21.95 it's not as detailed as the Scott catalog, but the numbering and values are exactly the same, so you can easily identify most US stamps that you have at only a fraction of the cost of purchasing an expensive Scott catalog. In fact, if you wait until late September/early October, the next latest issue should be on the market by then, so you'll have the most up-to-date information.
Going back to the percentage of Scott catalog values as secondary market prices, I should add one more thing: Many common stamps are shown in the Scott catalog at a value minimum of $0.25. The generally accepted idea is that is what the overhead cost is in a dealer having to keep that stamp in their inventory and a seller going to that dealer to obtain that one specific item. Just understand that should you have 1000 copies of a single common stamp worth the catalog minimum ($0.25), you cannot multiply the quantity times the value and think you have $250 worth of stamps; quite often the stamps valued at catalog minimum, which are typically common stamps, may be worth a penny (or less) in large quantities.
One has to realize that the Scott Catalog is only a Guide. The true market value of any item is what a seller is willing to sell it for and what a buyer is willing to pay. There are situations where a stamp can be "valued" at $100, but secondary market prices may suggest that the stamp is only worth $5 or $10 for resale, especially for relatively common material. The opposite can also hold true if you have an exceptionally nice copy of the stamp that is in exceptional condition or an item that is in high demand.
The same principle holds true when you are buying any other type of collectible, or even an automobile or a house. A seller can ask anything they want; it is up to the savvy buyer to know what the true value of an item really is and that typically determines a mutually acceptable price.