When I purchased a used tack item from the store, I accidently reloaded the page, and instead of giving me an error or something, it gave me a message saying "Oops! You cannot transfer tack between your accounts!" The account selling the tack was not mine, and I was able to buy another used tack item from that stable immediatly afterwards.

When the page refreshed, the game already accounted for the fact that you already purchased the tack and it's in your own inventory. You got the message because you aren't able to rebuy it from yourself. I believe the part about it being another account is just the default message the game goes to! :)