Many people reach a point where stress becomes part of their daily routine. They get stressed during rush hour on the way to work. They get stressed during crazy times at their job. They get stressed on the drive home, and sometimes even after they get home if there are family issues.
That stress starts to feel "normal" to them because it's like that all the time. That's what adaptation is- the stress is still there, but it becomes the norm. That doesn't make it less harmful though. It still causes very real physical symptoms: muscle tension and pains, elevated heart rate, rushes of cortisol, breakdown of tissues, and eventually advanced symptoms like a depressed immune system and more illness. Even more long-term, chronic stress is linked to cancer, diabetes, heart disease, and a much higher risk of developing Alzheimer's.
I actually worked in a research lab looking at the effects of stress for awhile, and it was amazing the number of people who had objectively stressful lives (caretakers and people with stressful jobs in particular) but who insisted that they felt fine and not stressed at all. Naturally, most of those people were much more likely to have the above physical symptoms, which they'd never thought to attribute to stress before. Most of them, when they worked on reducing their load of stress, say major improvements in health.
Being able to see ourselves as happy even in tough or stressful times is very adaptive, it gives us the mental energy to keep going. But stress is a very physical thing- the stress response you get is equivalent to the physical response you'd get in a threatening situation. If you're constantly responding physically to stress, your brain might start to see it as normal even as it's doing harm to your body.
Hopefully that was somewhat clear, I know both concepts are kind of hard to wrap your head around. As far as "buying something" goes I'm not sure what that's referring to? Purchases tend to create a very temporary physical euphoria, but they don't ultimately do much for chronic stress. The only way to reduce the effects of chronic stress would be to reduce the stress itself (by taking away the stressor or minimizing its impact on your life) and/or increasing your coping strategies, so that you don't actually get the physical stress response as strongly and can learn to shut the stress response down yourself. Most purchases won't really help you do either of those things.