Good points about bad fit (which shouldn't happen if they'd buy at a real bike shop) and about finding out that a shiny bike doesn't automatically make you able to maintain hundreds of watts' output power going uphill.
I have a shirt-tail relative who's wealthy and buys expensive things on a whim. He bought a high-end bike years ago, professional grade at the time, rode it very little, and after it sat in his garage for years, he gave it to me. It was the perfect size for my wife. It had professional gearing though (42/21 low gear, think Pantani in the mountain climbs), and she's definitely not a climber, so I had to make some changes in the components so she wouldn't be discouraged by hills. So because of his buying habits, we got an outstanding quality bike, free, and then I put probably a couple hundred dollars into it to get it ready for her to ride. She rode it a lot.
What about those who try expensive bikes and then decide cycling isn't for them? There's more to cycling than just a bike.
I have a shirt-tail relative who's wealthy and buys expensive things on a whim. He bought a high-end bike years ago, professional grade at the time, rode it very little, and after it sat in his garage for years, he gave it to me. It was the perfect size for my wife. It had professional gearing though (42/21 low gear, think Pantani in the mountain climbs), and she's definitely not a climber, so I had to make some changes in the components so she wouldn't be discouraged by hills. So because of his buying habits, we got an outstanding quality bike, free, and then I put probably a couple hundred dollars into it to get it ready for her to ride. She rode it a lot.