Face Value, Modern Mint, US Postage - all are terms used to describe the vast majority of US stamps issued over the past 75 years that are still unused and valid for postage.
There are some exceptions covered below but, in general, you can't sell these stamps at their face value so you're actually better off using them for postage.
The sad truth is that modern stamps are so plentiful that most stamp dealers only pay 50 to 70% of the face value if they're willing to buy the stamps at all.
Mail use is declining and more stamps are being sold into the market so supply is increasing and demand decreasing. The result has been a gradual but steady decrease in value of modern US Postage over the past few decades.
The bright spot here is that this means you can also buy US postage at a discount from dealers or online. With a bit of searching you should be able to buy some for about 70 to 90% of face value depending on condition and denominations.
Save some money and brighten up your mail with cool old stamps!
Another positive is that collectors can add modern stamps to their stamp collection for a fraction of the cost of buying them from the USPS and have a ton of fun along the way.
You can keep very busy just trying to collect stamps after 1940 as there are about 5,000 unique stamps recognized by the Scott Postage Stamp Catalog issued by the United States since 1940.
It is difficult to come to grips with, but many stamp collectors or those that have inherited a stamp collection, are finding out the hard way that an entire lifetime of stamps purchased at the local US Post Office are worth far less than what was paid for them (not even counting for the effects of inflation). This is an emotional realization but a fact of the stamp market nonetheless.
This really drives home a key lesson for all of us. Stamp collecting is a hobby to be undertaken for the joy of the hunt, the satifaction of building a collection, appreciation for art and history, or whatever your personal call to the hobby happens to be.
Buying stamps is only an investment if you buy the highest quality and extremely rare stamps - these are individual stamps in the hundreds of dollars to hundreds of thousands of dollars. I know I can't afford those...but it is good to have a goal so maybe one day!
It's not all bad news for the seller of modern stamps. There are some stamps that sell for way more than their face value so it pays to know what to watch for.
This is not a complete list but I'll hit some highlights and try to add to it over time.
1. Legends of the West recalled "error" sheet still in the blue folder (Scott #2870)
2. Bugs Bunny Imperforate mini-sheet (Scott #3138)
3. $5 Alexander Hamilton from 1954-68 Liberty Series (Scott #1053)
There are a handful of singles and sheets that are in demand somewhere between face value and one and a half times the face value. There are some pretty neat Space Hologram sheets, a Submarine booklet, and some other relatively scarce modern items that are in demand by collectors.
The big stamp wholesalers will frequently advertise higher than market prices for specific stamps - for a large buyer or to restock inventory.
Either way, you really have to follow the stamp buy prices very closely and be ready with large quantities of individual stamps to ship off before the buy limit is reached and the price drops back down to below face.
You'll also want to keep you're eye out misprints and production errors. Those can also have significant value if they are a recognized error and listed as such in the Scott catalog.