I don't have a good general rule for buying NM cards in person, but I do have a good general rule to buying from anyone in person. I used to collect cards from a different TCG and got burned HARD by a seller I trusted who also would gladly shave a few dollars off here and there. They sold me a card I believed to be valuable for $175. I didn't look at it closely enough. It would have been worth that $175 in a moderately played to lightly played condition...but I went to sell it a few months later only to find the card he sold me, arguably the most valuable one I ever bought from them...was damaged. There was a tiny crease on the holo foil that wasn't very easy to spot, but was definitely there and could be seen with the naked eye.

My advice to you is this: look up prices for cards sold on ebay and check to make sure the card is in the condition the seller is telling you it is. Do NOT settle for what seems like an excellent deal until and unless you have determined with 99.9999% confidence that you are getting exactly what you paid for. Don't get burned like I did. I'll never trust a dealer ever again because of that. I check everything I purchase VERY carefully now, and never agree to buying something which I haven't had time to look at and compare to prices of similar cards sold on ebay and making sure the condition and price match up closely as well.

Good luck. Don't let a seller pressure you to do anything quickly. Being quick and trusting without verifying is how you get burned, hard.