I saw some for sale and I can't afford them, but they had stats of like 150 or more and they were all level 0.

Super stats are max stats that have been bred to a pet with wild stats.
If you look at the numbers in parentheses, the first number is the base stat and the number after the + is the wild stat. Pets with wild stats are only found in the lost grove and the wild stats drop dramatically with each breeding, so there is a lot of luck involved to get a super with all of the base stats maxed as well as good wild stats.

Ok thank you! No wonder why they're so expensive!

to expound:
as said, the + stats always come from sylesti caught in the lost grove
> how high the stats are depend on the rarity of the sylesti
(Rarity meaning both the regular star rarity indicator + if the pet is themed or not (and now the new category of if it was obtained as an egg from the third section of the lost grove.) Each combo- eg rare unthemed, epic themed, temple egg- have their own bonus stat range, with legendary+mythical themed topping them off, having the highest possible, with a range of a whopping +50-+75 for the four base stats)

> clarifying, the stats drop each generation- HP drops by something like 40-60, Mana by ~10-20, and str/int/dex/agi by ~3-4
(If only one parent has + stats, it will take those then apply those drops, if both parents have them, then it will apply the usual 'pick a number between the parent stats' THEN subtract; so crossing a pet with +40 Str with one that's +0 will result in either +36 or +37, wheras parents that are +40 and +30 could end up with offspring + stat anywhere between +37 and +26)

Thank you for this detailed clarification. ^_^ It is quite enlightening.