Too many people make the mistake of thinking that the catalog value of a stamp is the final word. Nothing could be further from the truth. There are several factors that affect the selling and buying prices of stamps.
One factor is time. The Scott catalog does a good job keeping up with stamp values. However, such a huge catalog takes months to prepare and print. When the new catalog is published, it's already a few months behind the current market. That's one factor.
The prices listed in the Scott catalog are for VF copies with no faults. There are a few issues that are exceedingly rare and if the few copies known are in less than VF condition, Scott will add a footnote stating that the price is based on different centering and/or condition.
That brings us to our next factor, centering and condition. Take two copies of the same stamp, say Scott #245, the $5 Columbian. A copy that is VF and without faults will probably sell for near full catalog value, and perhaps even over catalog value. How much would you pay for a copy with only AVE centering, a major thin, heavy cancel, and a missing corner? Such a heavily damaged copy may only bring 5% or so of catalog value when offered for sale.
Another factor is demand. Many collectors need the $5 Columbian, Scott #245. Some will settle for a cheap space filler copy at a huge discount of catalog value. There is a market for this kind of material. How many collectors want a damaged copy of a cheap stamp that catalogs only 20¢? Almost no one. The stamp is so common and cheap, collectors generally shun damaged copies at any price. In my experience, damaged copies of stamps cataloging less than $5 don't sell very well. Once you hit $10 or $20 in catalog value, then damaged material will sell because there are collectors on very limited budgets who want to fill spaces. They will accept lesser quality copies.
People sometimes ask a dealer, "What percentage of catalog value do you pay?" I don't know of anyone that pays a strict percentage of catalog value. An offer depends on lots of things like the centering and condition of the material. Is this material the dealer generally carries? Is the dealer over or under supplied with this material? Dealers don't pay larger prices for items they are currently overstocked on. How quickly does the dealer think they can move the material being offered? How much duplication is there in the material being offered? For example, huge quantities of modern US issues in used condition frequently sell for pennies or a few dollars per thousand stamps. Even though 5000 copies of Scott #1597 may have a catalog value of $100, no dealer is going to be able to move that many copies that quickly. In this case, an offer may be a tiny fraction of catalog value.