Sure, you might make mistakes, and sometimes you might even get a great deal for yourself, but remember this is a hobby and if you want to make and keep friends, be up front about your extras and what you want to trade for. Suppose you collect stamps showing sports subjects and your best friend collects stamps showing ships. You might just trade them - even. And you have agreed to be really fair about this. These can be bought at a stamp store, or maybe you can find one at your local library.
Every stamp catalogue shows an estimated value for every stamp it lists.
And, since values can change, they print new ones every year. So, you look in the catalogue and find that one stamp is valued at 35 cents, and the other is valued at 70 cents.