It is not only forbidden to steal items. One may not help or encourage
others to steal. Included in this prohibition is purchasing stolen
merchandise, or merchandise that has very likely been stolen. The source of
this prohibition is the mitzvo of :"Do not put a stumbling block in front of
a blind man." This mitzvo teaches us that one may not cause or aid
other people to sin. This also applies to helping a non-Jew transgress one
of the mitzvos that pertain to him. Since it is forbidden for a non-Jew
to steal, one may not purchase an item that was stolen by one non-Jew from
another, because doing so, encourages the thief to continue in his evil ways.
Similarly, one may not sell equipment or provide information to a person who
appears to be preparing to use it in order to steal. There are two reasons
given as to why one may not cause others to sin.
1. To do so, constitutes a direct violation against Hashem. Hashem created
the world in such a way that the spiritual actions of human beings have a
great effect on creation. When a person performs a mitzvo, he causes great
benefit to the world in a spiritual sense. Conversely, when he sins, he
causes great spiritual damage. Accordingly, when a person causes or helps
his fellow sin, he is contributing to the subsequent damage.
2. Causing another person to sin also constitutes a violation in the realm
of interpersonal relationships (bein adam lechaveiro). When a person sins,
he not only damages the world, but he damages himself. It is not simply
that Hashem 'punishes' people for their sins, rather He has placed a
spiritual system of action and consequence. This system is identical to the
system present in the physical world. If a person steps off a high wall he
will harm himself - it is not considered as if he was punished, rather he
committed an action that had negative consequences. In a similar way, when
a person sins, he steps of a metaphorical wall, and causes himself harm.
Accordingly, helping someone sin, causes him harm, therefore it is forbidden.
 Much of the information for this essay is taken from "Halachos of Other
People's Money" by Rabbi Yisroel Pinchos Bodner.
 Kedoshim, 19:14.
 The Noachide Laws.