Most successful investors were knowledgeable collectors first. A well chosen, good quality stamp collection purchased at reasonable prices will appreciate in value, but probably no better or worse than any other investment. The best place to find out is at the reference section of your local library. There are a number of catalogues that will help, some for specific countries. When in doubt, the Scott Catalogue offers a fairly complete list of almost everything from everywhere.
Mistakes do happen and some get past quality control.
Most error stamps are worth a small premium over face value, although some spectacular errors can be worth thousands of dollars to the right collector. It depends on the stamps. Usually, auctions will get the fairest price, but not always. Dealers are also looking to buy stamps or collections. The best thing to do is take an inventory and then contact a few dealers and auctioneers to see if they are interested.