The postage stamp values expressed in most catalogs are a reference value only, in that they do not indicate what you will actually have to pay for the stamp or what you would get for it, if you sold it.
Retailers usually sell stamps at a substantial discount from the "catalog" price - sometimes 50% or more. As a result, dealers pay a percentage of their "retail" price of the stamp, to allow them to make a profit, thus the actual price you could sell the stamp to a retailer for would be a percentage of the retail value, not the catalog value.
Another thing to think about when evaluating a collection that you want to sell. Scott has a "minimum" stamp price of $0.20 in all their listings. This is intended as a guideline for dealers to sell cheap stamps by, in order to cover their handling time and shipping expenses. Many of these stamps, at $0.20, are the type of material that used to be found in stamp shop "penny boxes". As far as their wholesale values, they would have minimal, if any, value.
Many times, someone "inherits" a collection, looks up all the stamps in a catalog, and then determines that is what they can sell it for. This is not true. Many of these "inherited" collections from the 1940's to the 1980's contain common material. When these collections were originally assembled, whether by your parents or someone else, as a child, they probably had a marginal interest in collecting, did not have large sums of funds to spend on their collections, and probably got many of their stamps via penny approvals or packets of stamps. Unfortunately, these types of stamps are as common today, as they were 60 years ago.
Dealers have a difficult time selling this type of "modern" material. That is, U.S. after about 1935 and foreign after about 1940. The availability of such material is HUGE and there is not enough market demand to sustain sales of the material. On U.S. mint stamps after 1935, especially if they're hinged, dealers will only count them at face-value, when evaluating a collection for purchase.
On the resalable 19th and early 20th Century material that is in sound condition, the owner can expect a wholesale postage stamp value of about 10% - 20% of the "catalog value". Not that much out of line, when you figure much of the material may have been purchased at 50% to 60% of the catalog value.
If, by chance, there may be any rare items, with VERY LOW availability and VERY HIGH market demand, then that is another issue.