If you can put your hands on it before you purchase.. check the bore of course. Look for rust, pitting, and over all condition. Check the muzzle of the rifle close. Look for deep scratches, etc. Now **** the muzzleloader. If its a traditional, does the hammer come back and click through the half **** and full **** of the arch? Once the hammer is ****ed, slap the side of the rifle hard and see if you can knock it off the mark. Also check the trigger(s). Do they function properly. Is there a hard trigger, too soft a trigger, etc. If the rifle seems to function fine, now examine the stock. Your looking for hairline cracks. If you can pull the barrel, pull it and check the interior of the stock for hairline cracks or unusual wear patterns.
If you can not put your hands on the rifle, then you have to ask the seller about all the parts I mentioned. Get them to state... there are no cracks in the stock, the triggers, etc are all fine. The bore is not rusted. And if they tell you there is a little pitting or rust... WHAT DO THEY CALL A LITTLE. And where is the rust located. If you can get them to state condition, then you have something to fall back on if the rifle comes to your door and is in bad shape.