Be wary of having puppies and kittens for sale in your pet shop. We had them and it wasn't a problem when we bought in the puppies or kittens. However, people would turn up with a box of 4 or 5 week old puppies or kittens and say do you want these. We said, no, take them home for another month and bring them back when they are 8-9 weeks old. The following morning we usually had a box of 4 or 5 week old animals on the front door step when we got to work.
People also buy puppies and kittens because they are cute and then bring them back a few weeks later when the novelty has worn off and the people can't deal with the puppy poop and constant whining of the animal because it needs attention and isn't getting any.
We spent years trying to get the government to make laws saying you can't sell or give away puppies or kittens until they are 16 weeks old. We wanted this so people would be more inclined to buy an animal rather than "oh look at the cute puppy". It would also discourage puppy and kitten farms and make a few more people get their animals sterilised so they didn't have puppies everywhere for 4 months. Unfortunately it didn't happen.
A lot of shops around here won't carry cats or dogs but instead have links with the animal shelters. If someone comes in looking for a dog or cat, they show them pictures of the animals currently at the local shelters and send the people on to there. This stops people dumping animals on doorsteps in the middle of winter and helps find homes for unwanted or abandoned animals.
The other idea, which is what I would do if I was young and healthy and could be bothered doing anything, would be have a farm and breed dogs, birds & fish on it, and supply the shop with animals I produce. They would be small breeds of dog, good quality unrelated animals that didn't get sold until they were 4 months old. The young dogs would have basic training, learnt dog language and how to interact with other dogs ad people, vaccinated, dewormed and made available for sale as young dogs, not puppies. I would have open days for people to come out and look at the facility and maybe get a dog then or at least see what is available. This type of set up would make the dogs more expensive but it would stop people buying a puppy on a whim just because it is cute and fluffy and they are feeling maternal. It's a lot more work with no real profit, but I would do it for the animals rather than the money.