I used to work for that “State Line Tack in a PetSmart store” in NJ! Its great to think I may have helped some of you there in the past <3
From what I know:
State Line was originally a brick and mortar shop, and expanded. They were then bought by a larger dog/pet mail order company. PetSmart then bought that company.
After that, PetSmart operated State Line for a while and expanded sections into their stores. (I believe every horse owner has something like 2.4 dogs).
Then, State Line was sold to TABcom, its current owner. Its a bigger company that specializes in running niche online retail sites. They owned Dog.com and have picked up a lot of other online retailers since, including both Horse.com and State Line.