I don't consider a horse bought at auction to be a rescue
I do consider some. Yes, some kill buyers resell some horses, but you just need to see the feedlots around here, crammed full of horses being fattened for slaughter, to know that is a fate worth 'rescuing' from, particularly when you know they are crammed onto trucks, shipped to slaughtering facilities which by any definition are not nice places, then executed like cattle(no, I don't agree with the mass slaughter process of any form of livestock). For example, buying a cheap, healthy horse from an auction is not rescue. Buying an unhealthy, unhandled horse off the meat buyer is, in my mind. My BO bought a filly this last year, a weanling. She was the only one to bid against the meat buyer, and didn't pay much. The filly was very young, crusted in poo, very underweight, had an abscess on her chest and was unhandled. She was not a resale candidate for the kill buyer, and I doubt she would have survived a feed lot situation. she was, in my mind, a rescue.
To me, a rescue is a horse that has been removed from a situation(by the current owner, a horse is not a 'rescue' for the rest of its life) that promises an untimely death or severe physical or mental injury/harm. Thus the skinny thoroughbred that had a full round bale but needed grain was NOT a recue. Nor would the spoiled, reactive or poorly trained horse, nor the horse who's feet were a little too long, or was not on a deworming program, or who's owner didn't have time for them, so they lived in a large pasture full of grass.
when I think of an abused horse, I think of one that has been beaten harshly, tied up in a painful, unnecessary manner, or in other was unnecessarily mentally and physically abused. Some training methods are harsher than others, which does not necessarily equal abuse. Many people look at a horse that flinches or is reactive, and cries abuse. This is usually false. My BO has a mare that was born on her farm, out of her favorite mare, by her excellent stallion. Now, at 7 years old, she is a great saddle horse. From birth to 4 or 5 years old was another story. She was crazy reactive, mentally immature, and slow to catch on. Everyone who saw her thought she had been terribly abused, but in truth, she had never been treated harshly a day in her life. The term needs to be used carefully.
But regardless, as another poster mentioned, it should not make a difference in how you train and handle them. My arab mare was scruffy, wearing ill fitting tack and had shoes on that had probably been on for 4+ months. She reacts EXTREMELY to putting anything other than a bit in her mouth, to the point of her being willing to injure herself to get away. She had a similar reaction to getting shoes put on or taken off. Both the farrier and the vet, two men who are trustworthy, experienced horsemen not prone to sentimentally tagging horses as 'abused', told me that they believed she had been badly mistreated. That doesn't mean I treat her differently, or tell everyone she was abused. The fact is, I don't know what happened to her, and it really doesn't matter.