People lament buying things sometimes,
but let's talk about those that got away...

• Two kayaks with paddles for $100, in great condition, for sale at an auction at an inland, farmer-populated event, i there to get cheap furniture as a college student, with no car, knowing i already had to pay for transport of my stuff, would need to find storage space for the 8 months of the school year before returning to my coastal home where i could use them. Going, going... not gone. No bids, so they were removed from the auction (min. bid unmet).
• The collected works of Abraham Lincoln. 20+ books, in a used book store, for $75. Waa? This was pre-Internet so the extra value of having home access without libraries (heck, some libraries need that collection). I was in grad school in Windsor, Canada, just across the river from Detroit where the store was. But again, the hassle of housing stuff i couldn't use until the school year was up was the difference.
• The shoes that were so comfortable i thought immediately about buying a 2nd pair, but thought i could later. A year or two later i went looking and they were gone.
• The plane ticket to Malaysia for Christmas 2019. I decided to wait until spring (the end of my contract, negotiating a 2-week break in re-signing), and then COVID19 hit, hard. I haven't left this country since.

A decent mattress when I was in my 20s.

Huh. I guess you had it rough (lumpy?).

I took as an elective a Japanese Business Management course (in 1990 Japan was a darling of the biz world) and the prof from Osaka said we should travel in our youth because when you are older you'll demand more comfort. I heard it but it didn't resonate until my first full-time newspaper reporting job over five years later: The editor said one should travel and see one's country and the world while young because "when you're older you won't bother."

i regret not buying into particular older magic the gathering cards before the price spike (burnt that money on other garbage because i was impatient)