Due diligence may take a little extra time, but it can save a bundle of money if you discover that the stamp you have your eye on isn't what it seems.
First and foremost, be sure that the seller you're dealing with is reliable and experienced. Purchases from a reputable and knowledgeable dealer are usually the safest, and purchases should always be made in person.
It is safest to buy stamps from a dealer who has been in the business for a long time with an established reputation. The dealer should be a member of a national philatelic organization such as the American Stamp Dealers' Association, or the American Philatelic Society. Many countries and even regional areas have philatelic organizations.
Be sure the stamp's condition is exactly as the seller describes it to be. Look for the smallest imperfection-tears, creases, cuts, scuff marks, faded ink, missing perforations, folded corners, etc.
A stamp in poor condition should sell at a much lower price than a similar stamp in reasonable condition. Poor condition stamps are usually not worth buying at all as collector's pieces, even though they may make for attractive artwork or be used in craft pieces.
Before purchasing any stamp, particularly stamps that are described as rare or valuable, be sure you have correctly identified the stamp.
Two stamps may have a very similar appearance but have slight variations in color, watermarks, perforations, or paper, and such differences can greatly affect the fair market value. The Scott Standard Postage Stamp Catalog can help you in properly identifying a stamp before you purchase it.
For the average or novice collector, it can be very difficult to detect counterfeit stamps or stamps that have been damaged and then repaired. Large national philatelic organizations usually offer 'expertizing' services that can help determine whether or not a stamp is genuine.
But the fees for such services can be costly, so they should be used wisely. Except for very expensive stamps, the cost for detecting forgeries may not be worthwhile, especially to hobbyists or novice collectors.
The last consideration to be made before purchasing a stamp is whether or not the asking price is fair. Again, The Scott Standard Postage Stamp Catalog is a good place to look.
The catalog gives the fair market value of most stamps on the market today. However, the prices stated in the catalog are only general estimates, and most stamps actually sell for less than the price listed in the catalog.
If a seller is asking more than the price listed in the catalog, then you may want to reconsider working with that seller. Being careful; doing your research before purchasing used stamps is well worth the extra time and money.