I spent the 4 keys I had, and didn't get much useful. Purple and legendaries, but that doesn't matter if I'm not interested in them. I did get a cool shield, pretty high capacity (I have a large bonus too from some skill tree thing or another) and it has a pretty strong zap that shocks nearby enemies. Only 0.6s recharge delay too.
I got a silly legendary Torgue pistol that shoots these "roach eggs". The gun is slow to fire, does decent damage on its own, but the gimmick is that when fired at flesh (living or dead) the eggs hatch and the roaches damage enemies. It's not like you see these things flying around, you just see enemies taking a small amount of damage with those little hit feedback numbers. I'll be selling that.
What I was really looking for was a new Jakobs sniper and didn't get one. Ironically, there was a second gun vending machine on the other side of Carcadia settlement/base that had no "item of the day" but it was stocked with blue items. There I found a new Jakobs sniper, not the one I like, but another with one of those mock Aztec sounding names. A bit more finicky to use in combat, but does the damage that I need. I could have just bought that and not spent any keys