Is buying stamps as an investment a good idea? That, like any other investment strategy, will depend upon how committed you are to learning everything you can about the stamps you are wanting to collect.
Several others have advised to collect stamps as a hobby, and for someone just starting out, that is certainly very sound advice. The intrinsic value in a stamp is definitely in the eye of the beholder.
Now, can you make money with stamps? Yes, but it takes the knowledge gained from careful study and a willingness to look for them, then not spend more on acquiring them than what you could sell them for in the short term. This means you are not going to acquire a lot of valuable stamps in a short period. Over time, perhaps.
A few members talked about the viability of the stamp collecting hobby as a whole. I very strongly disagree with their assessment that the hobby may not exist in another generation.
What is happening is a paradigm change in the way the hobby is evolving. The cost of brick and mortar stamp stores has become prohibitive and few new stores are opening. Most dealers now work via the internet as the overhead is greatly reduced and the number of customers greatly increased. The same holds true for stamp bourses and shows.
It is easy to assume the hobby is dying, as the majority of collectors do tend to be in their 50's and up. There are reasons for that. As life circumstances change, with children finished with college and becoming established in their own lives, people in the older age brackets tend to have more time and disposable income. They are also often looking for something which is mentally stimulating and which is not as physically demanding. You can peruse stamp periodicals for the last 100 years and find knowledge and respected collectors and dealers talking about how the hobby is dying. I don't believe it at all, but it is changing.
For example, look at
ebay. On any given day, there are just under 1,000,000 U.S. stamps for sale, and many days, over that. If only 10% are in an auction format (and that is grossly understated), that means there's almost 100,000 stamps changing hands every 7-10 days. If we extrapolate that figure to a daily basis, and use the ten day term, that would mean 10,000 stamps per day or 3,650,000 stamps and collections changing hands yearly - and that is only for U.S. stamps based upon ONE site, albeit it might be the largest. When you add in worldwide stamps and collections, the number becomes even higher. It doesn't seem to me that this would be indicative of a dying hobby.
There are quite evidently millions of collectors and accumulators who purchase stamps via the internet, while never belonging to organized philately (members of local or national clubs and organizations). Does that mean they don't exist? Absolutely not, but the hobby is just now beginning to grapple with what this paradigm shift means to it. I am routinely outbid on many, many lots I'm interested in and am amazed at some of the prices collectors are willing to pay for the stamps they are buying.
Again, it becomes a matter of knowledge, and I believe everyone here would agree with me that the more you learn about what you are collecting, the greater the probability you will have a collection of value at the end of the road.
The other thing you have to keep in mind is what happens when it comes time to sell. If you have the time and wherewithal to list the stamps individually or as sets on
ebay (or another site with a large collector base, such as Hipstamp or Delcampe) you could probably maximize what you are obtaining. If you, or your heirs, decide to sell it to a dealer or collector, then understand - especially for dealers, that you may only receive 30-50% of catalogue value, and that value will normally be based upon the value of the more expensive stamps in the collection while a great bulk of most collections doesn't come into the consideration of the offer. If all you collect are better stamps, with higher catalogue values and have no interest in completing a collection, you would probably receive a higher return on your money.
At the end of the day, I believe that this is wonderful hobby, but it is a hobby. As collectors, we all have some residual value from the stamps we've enjoyed and the hours of pleasure adding them to our collections. Our heirs stand a chance at recouping some of what we spent, unlike Franklin Mint ceramic plates.
Hope this helps.