You're going to have a better idea of how an animal is going to mature if you hold them for a year. This is both positive in that it should give nicer snakes a fair bit more value, and negative, in that not so nice animals will barely be worth more than they were as hatchlings - or maybe even less if they've aged particularly poorly. I tend to hold back more than I plan on keeping to see how one matures against another. I personally find it easier to sell hatchlings and don't often sell yearling or older animals for much more. Admittedly, I'm terrible at advertising and tend to bump ads every week or two, or month or two, or never - depending on how preoccupied I get just raising up babies.