To be honest, I don't have any experience in this department, my shifting/cog has always matched, but carefully inspect all the tubes, to include the downtube all along the bottomside for ripples and/or dents. Do a quick look-see that the rear wheel is centered, then check to see if the front wheel is centered on the forks. Turn the cranks slowly, and feel for tight spots or clunks...........I really wish I was there, but just get a feel. If you get any bad vibes from the seller or the bike, just walk away. Deals can be had every day.