I’ve never been asked about having horses. It’s perfectly legal to buy and the store couldn’t care less if you’re buying it for yourself.
The expiration date on my new box is February 2023.
I can also buy it in my local Walmart, back in the pet department.
I have been buying it there (as well as Tractor Supply) for years, as I use it for my dogs’ heartworm prevention. Basically a couple cents per dose, as opposed to $25 for the Heartguard from the vet. The box and tubes of med are small, so you have to look for them.
My wife and I didn’t start taking it ourselves until last October.
DISCLAIMER - It tastes like CRAP. Now I know why I must trick my dogs into eating it.