It would seem to be a promotion for "members" who joined
ebay back in the 1990s. I would have to assume with some minimum threshold of activity.
I received such an email today. I am usually skeptical of such promotions, so I was inclined to forget it, but I did go back to it eventually after reading your post, Stamps1962.
I first reviewed the excluded categories mentioned in the email. These include but are not limited to: warranties, vehicles, real estate, gift cards, and of most likely relevance to Forum members, coins & paper money. Stamps are not mentioned as an exclusion.
Auction-style listings are also excluded, and you cannot apply this on a "best offer" item.
Edited to add: Contra what you heard, it is
not a one-day promotion. It is good until midnight Pacific time 10 September 2020. Also it is for items listed on US
ebay and both French and English language Canadian
ebay and the member needs a shipping address in the US or Canada.
I had been watching an particular non-philatelic collectible for a week or two, hemming and hawing 'cause it was a bit overpriced. It was a buy-it now item with make an offer enabled. I had been thinking of making an offer.
But with this coupon deal, I just went ahead and took it at the buy it now price. It was easy to apply the coupon which knocked off $25, which was well over half the price, and I settled for the remainder with Paypal.
So yes, it is real and works (from the buyer persepctive).
Now I just hope the doggone thing is not broken during shipment.