It's a little tricky situation, I am amateur in horses and owner thinks I didn't see that so I don't want mess things up with this. That mare is for sport she is in sport 7 years, she's walks normal and in sport is great. This is small photo of that foot it's hard to see it, but that bump is on the right side
Tricky? Well I don't know your circumstances it's true, but I would not be paying very much money for a horse with a bump/condition that I was worried about in the least. I would want to know more about it.
As I said, it could be one of many things. I would want to know more about that particular horses bump. Only way to find that out is to ask the current owner and to have a vet look at it. Not to mention poking around on it myself. ;)
Looks like a sidebone, which is from crappy shoeing/trimming or hard use in the past. It's essentially that the lateral cartilage (which is supposed to be somewhat flexible) has hardedned into bone. It can cause lameness, sometimes not. But is something to have a vetcheck before purchase if you really want to use her for more than a mosey around the pasture. If you can't afford to have a pre-purchase exam, you should probably skip over this horse, as it COULD be something that needs special care, and you can't know that without a vet to check for sure what's going on.
People can dope up a horse to show it to buyers or in certain events, so buyer beware, just because you saw her or were told she is fine, don't take their word for it, take YOUR vet's.