"Purchase" and "sale" are two different words in many languages; in Aramaic zbn can be used for either perspective, and there is little reason to assume (cf. DNWSI) that there might be different noun-formations involved in early dialects. By the time of Syriac, however, given the existence of the explicit D v.n zwbn, this form might be limited to the idea of "buying."
However, the number of noun formations actually attested is uncertain; see s.v. zbn#3 n.m.