This is kind of absurd! Height measurements can vary on the same horse. Because a horse's scapulas are not rigidly connected to their bodies, they can measure differently depending on the way they're standing, and how relaxed/tense they are. How they're holding their head and how long it's been since their feet have been trimmed can have an effect too. I have a measuring stick with a level on it. I've measured my horse a few times, while she is standing square on level concrete, and have measured anywhere from 16.3 to 17.1 hands, usually depending on how she's holding her head. She also sometimes leans away from the stick a little, so even if her feet are square, she's not standing 100% upright so I lose and inch or two on her. It's easier to miscalculate with a tape as well.
I don't think this buyer has a leg to stand on. The ad didn't even give a specific height. If it just said she was almost 15 hands, that's not a quantitative description and no promises were made.