Do you know the age of the bike? Do you know or can you find out if it is a steel frame bike? The Shoguns I have seen are 80s steel bikes. Most importantly, have you ridden it? Does it shift smoothly though all the gears and are the brakes good. Spin the wheels. Are they true? 150 is a little over $200 US and feels a bit high. But if the bike rides sweetened you put 2000 miles on it in the next year or two, whether you overpaid by 20 or 30 bucks won’t matter much.

I am assuming you don’t have an experienced bike riding friend who can help you check it out.