Cu2ZnSnSe4 and Ag2ZnSnSe4 nanocrystals were synthesized by a colloidal synthesis route and subsequently densified to form dense polycrystalline bulk specimens with nanoscale grains employing spark plasma sintering (SPS). Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron diffraction spectroscopy (EDS) were used to characterize the nanocrystals. The optical bandgap, thermal stability, and low temperature transport properties of the nanostructured polycrystalline stannites Cu2ZnSnSe4 and Ag2ZnSnSe4 were investigated. The transport properties of Ag2ZnSnSe4 are reported here for the first time and indicate polaronic-type conduction. Cu2ZnSnSe4 is p-type while Ag2ZnSnSe4 is n-type. The thermal transport in these materials is also investigated, the thermal conductivity of nanostructured Cu2ZnSnSe4 being greatly reduced compared with that of the bulk. Our results are presented in light of the interest in these materials for thermoelectric applications.