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Check with the other rider (gently) how she normally behaves, does she get strong? Do they just let her go as fast as she likes? Do they have certain places they always gallop? Etc. You may find they have inadvertently trained her to do this. There’s a big difference between being carted off with and a truly bolting horse.

Reschooling wise I’d just do 100’s of transitions every ride, walk to halt until she’s really soft and only then introduce faster paces. I’d probably keep it in an arena or small paddock initially too so she can’t get a head of steam built up.
 
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As @Jessie said there is a world of difference between refusing to stop and bolting, if you hope to sell her don't talk about her bolting because no one in their right mind buys a bolter unless they want a retired pet. Also if you could stop her with a one rein stop, even if it wasn't instant, then it wasn't a bolt - a bolter just doesn't stop, your aids are meaningless because the horse that was trained is no longer there and that's why they are so dangerous.

As well as checking what the rider has been doing with her consider how good a rider they are. It may be that they have a very different style to you that the horse has got used to and prefers, it may be that they are very good and have the mare more attuned to the aids and so you were confusing her - that's something I've seen many times.

Then there's the standard check her saddle still fits and she isn't in discomfort, that can make some horses reluctant to halt. Was it just going home, and so nappiness? Was there a common factor each time it happened?
 
Is that only ridden 10 times since you bought her or only ridden 10 times by you? If it's since you bought her then I'd guess a lot of the problem may be she needs more regular work.
 
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