The main thing I personally look for in a pet shop is one that sells high quality foods, treats and chew treats. I hate it when all a pet shop sells is the Royal Canine, Science Diet and Pedigree type of crap.
A selection of good highly rated foods like Ziwipeak, Orijen, Acana, pre-made raw foods, etc. (Some that are highly rated on the Dog Food Advisor website)
Good natural chew treats like dehydrated rabbit ears, beef tendons, bully sticks, tripe sticks, etc.
Other than that: harnesses (some that are small enough for chihuahuas
), leashes, collars, all kinds of toys (including little balls that are small enough for chihuahuas like Kong balls), nice clothes (including some that fit chihuahuas), beds, carriers, brushes, deshedders, nail clippers, bowls, etc.
Also products like pet toothpaste, toothbrushes, shampoo, certain meds that don't require a prescription (like for motion sickness or to help diarrhoea), supplements (like salmon oil), etc.
Like Susan said, the only thing I wouldn't want to see in a pet shop is the sale of cats and dogs. You can let good breeders and shelters advertise maybe, but the animals shouldn't actually be (or be sold) in the pet shop.