I feel silly for not knowing this but I have no clue how to breed with someone else's pet. I know that they are up for breeding because the heart is pink and the price is there and I can afford it I'm just not sure how to breed them ig. It always ends up saying "One or both of these pets do not exist." so..any help would be appreciated haha.

First you nead to have a pet from the same speacie with the opposite gender that is able to breed.

You then nead to select that pet in the chose pet section when you start a breeding with sombody else pet.