I would not pay $150 for that gun if the bore looks a little dirty. In fact, I wouldn't buy any gun without dropping a light down the bore.

In the case you described I would give the seller two options - either let you clean the bore and inspect it with a light before deciding, or take half the original asking price.

If you clean it and the bore is unpitted, $150 would be a very good price. If it's only lightly pitted, it would probably be an OK price. If it's badly pitted the gun is worth $100, but you could probably get it for less.