We report promising thermoelectric properties of the rock salt PbSe-PbS system which consists of chemical elements with high natural abundance. Doping with PbCl2, excess Pb, and Bi gives n-type behavior without significantly perturbing the cation sublattice. Thus, despite the great extent of dissolution of PbS in PbSe, the transport properties in this system, such as carrier mobilities and power factors, are remarkably similar to those of pristine n-type PbSe in fractions as high as 16%. The unexpected finding is the presence of precipitates similar to 2-5 nm in size, revealed by transmission electron microscopy, that increase in density with increasing PbS concentration, in contrast to previous reports of the occurrence of a complete solid solution in this system. We report a marked impact of the observed nanostructuring on the lattice thermal conductivity, as highlighted by contrasting the experimental values (similar to 1.3 W/mK) to those predicted by Klemens-Drabble theory at room temperature (similar to 1.6 W/mK). Our thermal conductivity results show that, unlike in PbTe, optical phonon excitations in PbSe-PbS systems contribute to heat transport at all temperatures. We show that figures of merit reaching as high as similar to 1.2-1.3 at 900 K can be obtained, suggesting that large-scale applications with good conversion efficiencies are possible from systems based on abundant, inexpensive chemical elements.