Don't pay too much attention to surface appearance, concentrate on the mechanical aspects of the guns. Look out for 'road grime' (dust, dried lube etc) inside the guns, however, that can mask their true mechanical condition. Get the one that is most functional mechanically. The bolt should reciprocate smoothly, with the 'drag' over the hammer being the exception. Note especially the carrier - it should snap up briskly as the bolt opens.
Having someone along who knows 870s well would help, if you know someone like that.