I assume the motive is BUYING them cheap to drink lots of coffee & fraps & food and selling them to make money! Some probably resell them on eBay for $85 (that's how I got my 1st one that got zeroed out by SB after about 6 weeks). What I find funny is when someone asks a seller "if the cards are legit." What's he gonna say, "NO!"
I have bought a lot of them. I use a lot of SB, sometimes 3x a day (they also "serve" breakfast and lunch sandwiches & salads). I also give some to family who also uses it. Maybe the actual sellers here are legit (and maybe not, who knows) but the people they originate from certainly are not. I look at it as a gamble. I've been very lucky so far but have absolutely no illusions on where or how these are obtained. If my $10 lunch costs me only $2 (I pay 20%-25% face value max) I'm happy. If anyone logically thinks about it, how could Starbucks sell a "legit" $100 card cheap enough to any of these sellers so that they can sell it for $20-$40 and still make money on it?
Here is an interesting article from last year (there are many more like it): http://gigaom.com/2013/08/07/how-scammers-drained-1700-from-my-bank-account-using-starbucks-cards/