Explanation of the following Mishnah is according to the Bartenura, Rambam, Kehati, and Tiferes Yisrael.
The Mishnah in Demai (2:3) discusses the requirements of a Chaveir (one careful regarding tumah and taharah). It says that he may not sell to an Am Ha'aretz either dry or wet produce, as he will come to make them impure, and one may not cause Eretz Yisrael produce to become impure.
Fine. So far, so good.
The next requirement is that he does not buy wet produce from them, as they would have become tamei in his reshus. Since tumah only rests on things that have become wet (Vayikra 11:38), the prohibition is only on things which were wet. Dry produce is entirely permissible.
Why are we not concerned that maybe it became wet in his property and subsequently dried? Whether the impurity touched it when it was wet or after it became wet it should still be considered tamei (see Rashi to above passuk). Why should dry produce be any better than wet produce?