Texto em português

Toda vez que outra criatura entra no campo de batalha, seu controlador pode comprar um card se seu poder for maior que o de cada outra criatura.

{G} , {T} : Adicione X de mana em qualquer combinação de cores, sendo X o maior poder entre as criaturas que você controla.

Rulings

Selvala's last ability is a mana ability. It doesn't use the stack and can't be responded to. If the greatest power among creatures you control is 0 or less at that time, no mana is added.2016-08-23
The new creature's power is compared to the power of each other creature on the battlefield as the first ability resolves. If another creature has the same or higher power than the new creature's power, no one may draw a card.2016-08-23
If the new creature isn't on the battlefield as the first ability resolves, use its power when it left the battlefield to determine whether its controller may draw a card. Note that effects that reduced its power before it left the battlefield will apply.2016-08-23