What happens to buying Google GC in person? WTB $150 google GC

Gift cards are typically invalidated in these situations and I remember reading about a guy who had Amazon charging his credit card on file for the value of items bought with the stolen gift cards.
Having said that all gift cards sell here on a discount- that doesn't mean they're stolen. It's typically people selling gift cards they won at raffle, gifted, etc. Look up the sticky thread for advice.