So let's say that hypothetically I purchase an ebook online and later on find out that the site/seller was likely selling a pirated copy. Now I don't condone piracy, so I'm asking if I could be in legal trouble as the buyer, in this hypothetical situation, despite paying for it and not actually actively seeking out for such a thing.
From what I remember from law class, he's an example: if you bought a stolen car, you wouldn't be in trouble for steeling the car. If caught you would have to return the car and could sue the seller, for the amount you lost/recoup your loses.
While each country has different laws, in general, if you buy a pirated copy and you didn't know that it was pirated (this includes willful ignorance) you won't be in trouble. By 'willful ignorance', you specifically chose not to concern yourself with finding out whether or not the copy is legitimate, meaning, you don't know it's pirated, but you don't care enough to find out if it is indeed pirated. But if you do know that the copy is pirated and you buy it anyway, you won't be charged with copyright infringement. However, the copyright owner can still sue you for assisting in copyright infringement. They likely won't do it, though, as copyright lawsuits can cost as much as half a million dollars if it goes to court. Regardless, Ai-chan doesn't know enough about all the copyright laws of every country in the world, so don't take this as a legal advice.
no. since you didn't actively seek it out knowing that it was pirated, you don't have any responsibility