Depending on the kind of performance you are looking for from a canoe, hogging need not be a deal breaker. It makes the boat a little or a lot slower and a little or a lot harder to turn, depending on how pronounced the hogging is. Plenty of people paddle hogged canoes and are satisfied with how their boat works for them. A discerning paddler with specific expectations regarding boat performance would not be happy with such a boat, but most canoe buyers are the complete opposite of that.
Hogging can be rather subtle, and any degree of upward bending of the hull into the interior of the boat is considered hogging. Further, hogging will generally be worse when the boat is in the water with paddlers on board, or the hogging will appear at such times and yet not occur when unloaded or not in the water. Weight applied to the seats is counterbalanced in large measure by upward force on the bottom of the hull, and if the hull has any flex, the central parts of the hull will bend upward due to that buoyancy force. When inspecting the boat on dry land, pushing on the bottom of the hull with your hand will give some indication of what to expect when it’s in the water and loaded, at least if you are used to seeing hogging happen under load.
Pblanc will have better answers than I about what other problems to look for, but in general, poly boats are practically indestructible and if you don’t see damage, the boat is probably “good enough”. If there is damage, poly is very difficult to repair. Scratches on the bottom usually are no big deal and are not considered “damage” unless there’s been severe wear.