What Is A Reasonable Percentage Of Cat Value To Sell USA Stamps ?

I like the 100,000%! (When selling)

One question is how fast you do you want to sell them? The higher the price the slower you will sell them and your access to buyers. You could sell them for a fraction of CV but if no one knows you have them for sale you will not sell them. Some dealers make a living selling at CV because they advertise or have customers. They may offer other products too.

In order to get access to buyers (ebay, hipstamp, etc) you end up having to pay listing fees. There are some stamp forums that organized selling setups.

What US stamp goes for 100,000% of catalog value?

Some common stamps, if in a very high grade, can sell for very high percentages. For example, Scott 551 (1/2c Nathan Hale 4th Bureau), valued for VF80 NH is 50c ($0.50). But in Superb98 NH is $140.00 (at least in my 2014 Scott catalog), which is 280 times the VF80 pricing.
This is a 28,000% of the VF80 valuation.

I have actually sold this stamp for 8000% of the catalog valuation (that is, $40.00).

-dave

Spot on Dave. When I assembled my 4th bureau set many of the stamps I paid well over catalog value for the pristine pieces.

A newbie's question: If a realistic price is 10-20% of the catalog value, whom does the catalog serve? Where is the CV applicable?

That's just it, 10-20% isn't a universally realistic price. There is no such thing unless you're buying seconds, packets, or kiloware.

Full catalog price is perfectly reasonable for stamps that are in the specified condition Scott sets for the issue. If you buy from a reputable auction house you'll see things selling close to catalog when the condition is close to what Scott specifies, lower when poorly centered for with faults, and higher when better centered and pristine. Obviously supply and demand changes and so prices also change all year. Catalog prices are very helpful if you are careful to understand what they are intended to mean and know how to evaluate the stamp you are looking at and know the market you are in.

I find that I can price a stamp entirely by looking at the catalog value and the stamp condition - take off for centering, take off for condition issues - and I get a price that is generally competitive. If I check other people's prices and see something different I will adjust for the market. I will only price at catalog for a pristine stamp that matches the condition specified for the price in the catalog.

Summary: price for a damaged stamp will often be 10% of cat. price for a great stamp may be about 100% of cat, price for an amazing stamp can be double or triple cat. Cat provides the baseline off which one can figure out the value of the stamp you have in hand.