The biggest knock against it that is keeping it out of Constructed is its 3 toughness. While 4/3 isn't terrible for a five mana creature that can easily get bigger, Stoke the Flames is all that it takes to take it out. With Exquisite Firecraft from Magic Origins, another burn spell that deals 4 damage, now in play, things don't look much better for this Regent.
But Firecraft is not as easy to cast as Stoke the Flames... So when the Magic 2015 instant with Convoke rotates out, the Regent won't have to deal with that. Then again, there's Languish in Magic Origins, which gives all creatures -4/-4 until end of turn. Basically for Sunscorch Regent to see any meaningful Constructed play, it has to consistently reach 5 toughness. That being said, it will gain a counter in response to the removal. So, Stoke the Flames being gone definitely helps its case.
So there is some place for this Regent to go in an upcoming Standard format. But what's driving the Amazon sales? Looking at dealer buy-lists, this seems to mainly be an Amazon-only phenomenon. The most curious thing is that the Regent buy lists for a quarter, which is above bulk. But the foil also buy lists for about a quarter.
It is a Dragon and it fits into a number of Dragon Commander decks. It also fits into strategies that love +1/+1 counters. I think of all the Regents, this will be the third of the cycle to emerge, after Thunderbreak Regent and Icefall Regent (which still has more room to grow). It just needs a standard rotation to see a rise in value. It's definitely a card to watch, and.the casual crowd is clearly ahead of the game on this one.
(And there is this B/W Revelation Control deck that runs 4 copies of the Sunscorch Regent. So there is definitely room for this card to do some damage.)