There are flats and there are books. Flats are a sheet of 20 Forever stamps that you would get at a drug store. A book is 20 stamps, used or not used.
Generally there are different values associated with flats because you can get cash for them on the streets. You can get them to the streets via an officer if you're ballsy. Or you can just find discreet ways to mail them.
The value of a flat where I was at was about $13-$14 depending on your vendor. A book was generally worth $10. If you were buying bulk (and depending on who exactly you were buying from), that number could go down to $12 and $7 respectively.
If you were buying from someone that was running a ticket (sports gambling) they had more incentive to get rid of their stamps. Some people had tens of thousands of dollars of stamps in their property and they wanted to convert them.
If you were buying from someone running a store (think "buy and resell commissary"), their business model was dependent on flipping books of stamps in exchange for more commissary. They could sell a ramen for 3 stamps ($1.50) when it was worth just 50 cents.
Flats were easier to hide because they are, well, flat.