Tin selenide (SnSe) has notable thermoelectric properties, yet stable n-type polycrystalline SnSe is difficult to synthesize. Here, polycrystalline SnSe is easily prepared by arc-melting as robust pellets, with thermoelectric properties repeatably changing to negative Seebeck .coefficient above 580 K reaching a figure of merit similar to 1.8 at 816 K. DC conductivity changes 4 orders of magnitude with temperature, whereas microwave conductivity increases only 4-fold, confirming the effects of oxidized grain boundaries. Effects of ambient oxygen exposure are probed by X-ray photoelectron spectroscopy. Neutron powder diffraction reveals 3% Sn deficiency. Inelastic neutron scattering shows phonon spectrum consistent with ab initio calculations and reported Raman spectra, but with higher-energy modes strongly softened at higher temperatures. We thereby provide insight on undoped n-type polycrystalline SnSe that reveals high-performance at high temperature, being a suitable peer material for p-type SnSe.