I also think this is legitimate simply because its been a year since he's been doing this and I think that if they were from an illegal source at least once person would of reported being contacted by the police or home depot to report that the card they used was acquired illegally.
Just for what it's worth, let's pretend someone was selling store-credit cards acquired by returning shoplifted merchandise for credit. Just to make it clear, I am not accusing the OP of stealing the cards offered for sale, but simply addressing the potential for fallacy of the conclusion above.
Home Depot doesn't ask for your contact information when you buy something in person with a gift card, so unless they can stop and talk to you right then and there, they're never going to contact you.
Agreed they get your contact information when you buy something online. But if they know the card has stolen funds, I think they would just deactivate the card before you had a chance to use it. It's pretty unlikely that they would leave it active, wait for somebody to order something online, and then send the police to your door, without any evidence that you (as the user of the card) were the shoplifter who originally acquired it, or even the person who placed the order online. It would be an enormous publicity disaster for them, they wouldn't even consider it.
If a shoplifter is caught stealing and returning merchandise for credit, he is probably going to be caught in the act. In that case, they will surely retain any merchandise or cards in his possession during the bust. But if he gets caught on his 10th time after successfully leaving with good store credit 9 previous times, how is the store going to know that, let alone know the serial numbers of the cards he received in order to deactivate them? Simple, they aren't, at least not immediately. So those cards are going to appear "good" and remain "good" indefinitely. The fact that the cards remain good doesn't prove in and of itself that they were acquired legitimately.
It is always possible that law enforcement or the store might get lucky during their investigation and figure out serial numbers of stolen cards (e.g. through surveillance recordings, or by coming across an unexpected opportunity to seize the original cards still possessed by the shoplifter much later, such as from his home, or if he is persuaded to hand them over in an effort to cooperate or mitigate his punishment.) If this happened, it would almost certainly result in cards that mysteriously stop working for reasons the Home Depot customer service will refuse to explain.