Seriously?  If someone walked up to you on the street and said to you "I have a $100 bill I can sell to you for $20" would you ask that same question?  What do you think the other person would answer?  "Nahhh, they are counterfeit."

That being said, the way to resolve your concern is to ADD it to your account as a separate card rather than add it to your existing card.  If you pay with your phone app you just select the card you want to use. Rotate between the cards. They won't close your whole account just the individual card. (I have 6 cards on my account right now).  DO NOT merge/top off them with your LEGIT card (legit=the one you paid full price for).

   If you REALLY think any, not just from this guy, of these cards are legit you have to be hallucinating. Do you think Starbucks is selling all these sellers $100 worth of product for $10 so they can sell them and make $10 each?   I've bought a bunch of them ($800+) over the last few months (NONE FROM THIS SELLER) and have had only one problem.  Spend the first $20 on each card real quick and anything after that consider it a win.  If the cards are still good after 2-3 MONTHS then you should be safe merging them with your regular card but I still wouldn't do that.  

Are they legit?  Yea, sure, wanna buy a bridge in Brooklyn NY?   Do they work?  For the most part I've been quite lucky. YMMV