The determining factor is how they were dried and stored, and it's a pretty safe bet that your dealer didn't devote lots of time to making sure the mushrooms stayed in mint condition. Since these mushrooms will be weaker, you'll need to buy many more than you normally would need to consume to trip, and it is more economical in the long run to become self-sufficient. Even if you decide to preserve them, you can do so in a manner which won't impact their psychoactive compounds much. Mushrooms have been used for thousands of years as a sacred tool for enlightenment. I personally find it far more satisfying and sacred to consume a mushroom which you nurtured yourself.
Dealers are for drugs like speed and heroin.
Growing mushrooms is easy. Shroom hunting in the wild can also be very productive, but unless you know exactly what you're looking for, you could accidentally pick and eat, oh, say an Amanita Phallodies mushroom which would cause liver failure and a slow, painful death. The benefits of shrooms hunting, though, are that you don't need to spend a cent to do it. At any rate, it can also be most satisfying to stumble across hundreds of fun-guys just sitting there in a pile of cow excrement.