First thing I'd do is look at their traffic history if possible. Look as far back as you can and see if there are significant drops in traffic. It would be a good sign if there were not any significant drops. If the terms they're getting their most traffic for now were the same terms they were getting their most traffic for two or three years ago--and they rank down on page 2 or 3, it could be a sign of fixable under optimization.
If, however, you don't see any significant drops and you examine a page (maybe their homepage) and it looks appropriately optimized for appropriately competitive terms but the page is not showing up anywhere in the results, I'd hesitate. There could be a penalty far back in it's history that you're not able to see.
Take a look at the wayback machine and see what the site looked like prior to the analytics history you're able to view. Was it the same site? Maybe the people you're buying it from bought it from someone else who'd spammed and jammed it. If that were the case, maybe a simple reinclusion request could fix the problem--if there is one.
There are a lot of possibilities and they're all worth looking into. Of course, the smart buyer might do exhaustive research, keep the results close to their vest, and push the seller hard on the lack of traffic issue to suppress the price.