Horses often change personality in response to new riders. The seller may have done nothing wrong. A lady who used to live about 1/4 mile from here went thru a dozen horses in about 3 years...all of whom turned out to be 'bad horses'. The farrier lost her business when, after 3 years, he suggested it might be her!
It happens. It happened to me. Mia was sold to me as "perfect for a beginner". A trainer a few years later suspected Mia had never been broken to ride. But I am certain many of her problems were caused by my riding. Riding her bitless didn't help. She is one of those horses who really need a bit so she can feel comfortable that her rider is in charge. She also desperately needs a confident rider, which I was not and still am not.
None of that would make you a rotten person or rider. Some horses truly just have personalities that make them good to learn on, but it is very hard to find them. A more demanding horse will take advantage of what you don't know, but behave beautifully with a more experienced rider.
Legally, I don't see where you would have any reasonable cause of action. If it went to trial, the previous owner could easily find experts who would honestly testify that some horses behave well with one person, and not with another, and that it is hard to predict. Heck, Trooper adores my daughter and takes care of her, but barely tolerates me. Mia will throw my daughter, but do her darnedest for me. Why? Don't know.
It wouldn't be wrong to sell the horse. You will take a loss. Join the crowd. I've lost money on every horse I've owned...
There is a reason this thread is a "sticky": "Playing the Hero" -- when to 'stick with it' & when to realize it's time to move on.
BTW - I'll add that Mia dislikes girls. If your daughter tried to ride Mia bareback, Mia would almost certainly throw her. Heck, I'm not sure she would tolerate ME bareback, and I've had her now for 5 years.