If you are buying 'raw' ingredients, such as mushrooms or marijuana, there is a much better chance of getting what you expect.

Of course, it's not guarenteed (they could for example be sprayed with another substance), but even in terms of economics for the dealer, it probably wouldn't make sense to try to adulterate such things. Adulteration is much more prevelant when dealing with relatively expensive, highly processed drugs, where there are many levels of people in the supply chain; the temptation to cut it to increase margins is just too high.