I've told this story before, but it answers this question directly:
For me, one purchase easily was the 90-degree hi-fi turn in my life. Somewhere in the vicinity of 20 years ago, I was still interested in having good electronics and hi-fi equipment, but I was happy with the following:
A 55-watt 2-channel Yamaha receiver
(2) Optimus bookshelf speakers
(2) no-name bookshelf speakers that I put on top of the Optimus to make it look like a tower speaker set-up : so cool was I
I was satisfied with the set-up. It wasn't audiophile quality, but I was a young parent with a young kid, living in an apartment scraping by. We had saved money to take a trip to Florida to visit relatives, so we spent a week down there. A couple of days before we left, one of my Optimus speakers blew, so I knew when I got back I would have to do something. We spent wisely in Florida, and I came back with a little over $500 in my pocket. My wife actually sent me out to replace the speakers.
I worked for my father-in-law as an accountant (now CPA), and one of our clients was a hi-0end audio/video store. I always loved looking, but couldn't afford much of what he sold. Well, I figured that would be my first stop, as the wife asked that I keep my purchase to around $200 for new speakers.
As I pull up to the parking lot of the store and get out of the car, there is this gentleman walking into the store holding a fairly large speaker. I follow this person in, and the owner greets me at the door. As always, my curiosity gets the better of me, and I ask what's up with the guy that just came into the store. The owner says something like this:
When rich people move, they need boxes to pack things. Instead of finding a box for these speakers, this person brought in the speakers to trade up to the next model that were already in boxes. I come to find out the person was Garin Veris of the New England Patriots, and the speakers were ADS-1290 towers. I can't remember exactly how much a pair cost back then, but I'm pretty sure they were somewhere around $2,000. I also come to find out the speakers were only a little over a month old with no use. So of course I must ask - how much do you want for the speakers. The owners response was "How much do you have?" So I pull the cash out of my pocket and start counting - $100, $200, $300, $400, $500 - GRAB. The owner takes the $500 and says take the speakers.
They barely fit in my Toyota Corolla, and man-o-man the look on my wifes face when she came in the door that evening and saw these tower speakers. Of course she was mad at first until she heard them.
Twenty years later they are still my favorite item of all of my audio equipment, and they started my meteoric rise to owning stock in Tweeter Etc (not really, but the amount of money I've spent at Tweeter deserves a stock certificate) and having more audio/video equipment than most people I know.