It's like this. If you have a gift card for $50 at Target, there's no way to transfer that for a $50 gift card at Best Buy. The two companies have no incentive for letting you transfer funds back and forth.
If someone says "mail me your $50 Target gift card and I'll mail you my $50 Best Buy gift card," it'll work as long as they are honest and you get your card. But if you mail them the card and they don't mail you one, you're screwed. Target and Best Buy aren't going to enforce your side deal with the guy, so you're out of luck.