Hi Ikuna, John's advice is good and you would be well advised to follow it. Even if you get a Scott/SG catalogue which is a couple of years old then it will repay you ten-fold when you buy. Stampworld and an old but recent catalogue will give you a very good guideline to current prices. If they say it's cheap (say, less than $25), then it will have remained cheap. If they say it's expensive then you should do more homework to find out current prices before buying, because some markets are very buoyant at the moment. All online auction sites have a "prices realised" section, and a cautious scan of eBay
will also give you some idea of what a stamp of "worth" to somebody.