Well - if you don't know what you are buying and they do not know what they are selling is one thing. But I don't think it is cool to take advantage of someone just because they do not know what they are selling. You should always make a fair offer - especially if you know what you are buying. It might not be so cool if you were on the receiving end. Just always try to be fair. Now there was one coin I bought from a local dealer, he priced it about $100 to $150 less than it should have been priced. I did not know it and I bought the coin. I caught some flack on here and it was the way I posted it the thread. The next time I was in I asked him about it - his reply was: No problem I will make it back in the long run. That was okay - but now when I buy from him I on some coins I asked and double check the prices. On some well know varieties I always double check the prices. On a few I have told him it was cheap for the variety and if he is selling I will buy it. He even encourages cherry picking - he has told many on his large cents to pick away. As it is the pictures are not large enough to really judge authenticity, but it looks legit to me. As mentioned try the magnet test.
The thing I like most is the S mint mark. San Fransisco stopped minting coins for circulation, so that makes it more interesting.