well that's easy, just make that having another egg for the same pet useful, e.g. using same egg increase the corresponding pet's level,

anyway, are you really selling eggs? who buys those eggs anyway? aren't they usually a 100% new player? in your case, do you really enjoy leveling the same pet, again and again? 0_0a

well, things that can be farmed anytime are meant to become less expensive by time anyway... and if it is really implemented, seller can increase their price, because the worth of one single egg is also increased

so yes, please combine pet stable