Disappointment When Goal Fulfilment is Delayed
I can think of a term, but it's not psychology-specific: unmet expectations (that and lack of attention).  Your two case studies are basically the same in these aspects.  

Unmet expectations are just that: we expect a certain emotional state when a situation is achieved (like buying a new car) - if that state isn't fully reached (which it rarely is - life is so rarely as clean cut as our minds would have it), our expectations are unmet and we are left 'to make up the difference', so to speak - disappointment, anger, frustration, all of these are possible, and more.  

How important we rate the outcome is also important - and unmet expectation isn't a big deal emotionally if the outcome isn't very important to us.  This is similar to reading a book and then going to the movie adaptation of it: if you have a mental picture of how the movie 'should' look, then you may be disappointed with how the actual movie turns out.

The other half, not enough attention, is also quite straightforward: if we are paying attention (the right attention, of course) we can see that material possessions rarely bring lasting happiness (this is actually a function of a remarkably adaptive human physiology, but I don't understand it well enough to explain it over the internet, sorry).  And with attention, we know that while we may enable more experiences, fun or lasting experiences, with material possessions, pursuing the thing itself as the end, rather than the means to the end, will inevitably result in dissatisfaction.

Lastly, there are a lot of popularized articles about expectations.  Here are a few:
Psychology Today article
NCBI Paper: The effect of unmet expectations among adults presenting with physical symptoms.
Tiny Buddha: Unmet expectations

Most of these were found by Google Search for phrase unmet expectations, so there's a lot more where those came from.