You'll be able to post your piglet by clicking "auction" when you buy, or give birth to them.
You can't post your piglets after you buy them or when the piglet is a "waiting pig"
24 hours will be the bidding time after you post them.
Once the bid is over, you'll receive post points.
If it is not bidded, your piglet will go to an open spot in your pigsty, or it will be brought to the "waiting pigs" list
Those pigs will automatically be brought into the pigsty once there's space.