Part of it will depend on if you are marketing to dog owners whose dogs are general pets, or to horse owners who have farm dogs/hunting dogs.
For either group:
Cotton-web leashes – 6 ft and 15 ft. Much easier on your hand than nylon, less bulky than leather.
Training collars – good quality
ID tags
Bitter Apple
Simple Solution Pet Odor Remover – enzymatic cleaner
CET Virbac toothpaste
Good shampoo – Chris Christensen
Cool mats (gel filled)
Squeaky toys – all kinds
Stuffed toys
Toys that have/can be refilled with plastic bottles
Kongs, Gummabones, Nylabones
Bully sticks
Smoked bones (NOT pork bones! supermarkets sell them but they splinter easily)
Vet wrap (which you would have on the tack shop side, I’m sure!)
Paw booties for injuries or protection (thin vinyl ones)
Health items such as dental care stuff, over the counter meds like kaopectate, ear care, wound care, first aid items, anti-itch products, cone alternatives like soft cones
Supplements – especially for skin and coat, joints
Grooming clippers (yep, Andis or whatever you would have for horses)
Dremels and nail clippers
Underground fence kits
Bark collars
If you want boutique-type stuff for pet owners:
Bandanas
Fancy flat buckle collars
Fancy treats
Treat pouches to clip to a belt or waist
Night visibility items like LED collars and tags
Dog food storage containers
Cat water fountains
Door mats or placemats for food bowls
Collaspible soft nylon crates
Car seat covers
Belly bands (for males who mark, usually toy breeds)
Panties for females in heat
If you have hunting dog owners (dunno, but maybe…):
Retrieving bumpers
Ecollars (not the cheap ones)
Skunk smell remover? LOL
Anything in bright orange or camo! 
NOTHING by Hartz!!! :dead: LOL