As you're chatting with a buyer, you may also choose to invite them into a video call. This will allow you to show them the pet live, vet the buyer, and observe the environment of the buyer. It's the next best thing to being there in-person!
On your chat window for a listing, you can see the video calling icon on the top. Once you've tapped it, it will invite the buyer to join in. Once they join in, you should be able to speak to them and see them. In case they do not join, they would have missed the call but, they would still have received a notification of your attempt to call them.
In case they make the call from their end, you will get a notification on the app to join the video call.