Philb and khj together do such a great job describing how things work that there is hardly anything left for me to ramble on about.
First, welcome The_Hoff.
The stamp buy and sell business is just like any other business when you get down to the nitty griitty. If you can expect to sell for a certain amount then in order to make money and have fun along the way you must be smart and buy for less than you would sell for plus a bit to cover your time and supplies and coffee and donuts etc.

Scott and other catalogues give you a rough relative value compared to other stamps. Many people think that the value in Scott or others is what a stamp (any copy of it) is worth. Nope. The catalog values are what you should expect to roughly pay for a really nice copy of that stamp, buying them one at a time, from a dealer that needs to cover his costs to keep the roof over his head.
Scott is not a black book kind of thing for stamps. The human mind gets carried along in it's wants and desires and I know when I first started looking at catalogues I thought, boy this will be great, look at all the money I can make here. Oops.
First you have to buy the stamps (or
inherit them as many do), then you have to spend lots of time sorting through them, checking them in catalogues (if you are not familiar with stamps), valuing them, finding people to sell to, dealers, collectors,
ebay, etc, doing it all over again and again and you had better like this or you won't have fun.
Depending on a lot of factors: the economy, the supply and demand of a particular stamp, the desire of collectors in countries with high gross national products to collect that particular country or topic or quantity, the actual condition of the stamp, how it is presented to the buyer, your salesmanship skills, the seeming scarcity of a stamp and probably more factors, you will arrive at a price that that stamp or stamps can be sold for.
Example: You have a sheet of Marilyn Monroe stamps, mint (with gum as issued, seeming fresh from the Post office). Good topic. Lots of people like Marilyn or the image of her or the spirit in the idea. So, you have some collectors who would want one in their collection. Not all. Some maybe would want to collect the sheet. Not all. Among those collectors are some who don't have the budget this month or year, some who have other pressing obligations, some who are interested in another subject or acquisition right now, etc.
Among those left, who see you selling this item, or among the dealers that see more collectors than you would ever want to see probably, there are some who are better at getting stuff for cheap, others who are more or less fair and the rare one who wants to wallpaper his room with the things and will pay any price almost.
So, you might sell one or two for $1 each, less your overhead costs to people who just love it and have to have it. Maybe. More likely you can use it for postage or sell it for 75% of the value on the face of the stamp. If it's old, that doesn't matter. If it's scarce or in demand , that matters.
Stamps do not have an intrinsic value like money does, for example. You 'buy' a one dollar bill at the bank for $1. Ten years later you sell it for at least $1 and perhaps more. A dollar is mostly always worth a dollar.
Stamps are totally valued on demand and the buyers perception of their value. The catalogues are there to help the dealers and the collectors. They are a sorting out tool but also a selling and buying tool. Not really a guide, a tool. A difference.
Stamps are sold by governments to show that a service has been paid for before hand when they are affixed to a piece of mail. The service of handling and delivering the mail. You are paying for the service, not the actual physical value of the stamp. But because stamps are nice and sometimes pretty and also appeal to the collector is some of us (and the would be investor (!?) whether he be well informed or not) they have a market.
Stamps buying and selling and collecting can be a fun hobby and business. There are a lot of nice people involved in stamps.
Do some research (well, you are already by being here), perhaps go to a stamp store or stamp show or bourse, look on
ebay or Delcampe or Wensy or BidStart (all dot com) and see what things are selling for. Not what prices are asked, but look at the completed listings or finished sales and see what actually sold. Not everything sells all the time. More stamps are sold in the fall and winter usually. Stamp season.
Depends on what you're up for, getting rid of the things, learning and studying and then selling slowly, dealing with people, selling, storing, sorting, cataloging, guessing at a sale price that will move the stuff etc etc.
I have rambled on long enough I feel. One last bit, I have figured out by magical means and consulting the bird that flew by to the East and divine intervention no doubt that to sell a stamp or to get people interested in buying, if they know the stated catalog value or have a rough guess at it, then you must price the thing at no more than 22.5% of the cat. value to start with. The higher you price it the longer it will take to sell.
edit: typos and spellings.