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Of American-English origin, the expression buyer’s remorse (also buyer remorse and buyers’ remorse) designates a feeling of regret experienced after making a purchase, typically one regarded as unnecessary or too expensive.

In extended use, this phrase designates a feeling of regret experienced after voting for a particular person or programme. The following, for example, is from Is this just the beginning?, by Andrew Coyne, published in the Montreal Gazette (Montreal, Quebec, Canada) of Saturday 25^th June 2016 [National Post: page 2, column 6]:
—This text is about the referendum held in the United Kingdom on Thursday 23^rd June 2016, and the consequent withdrawal of the United Kingdom from the European Union:

It is an extraordinarily bleak corner the British (or rather the English) have painted themselves into: a deteriorating economy, a disintegrating union, with no guarantee of either the trade access they were told they could keep or the greater sovereignty they were promised they would win. At some point, possibly before long, it is possible more than a few Brexit voters may come down with a severe case of buyer’s remorse, or Bregret as it is already being called.

These are, in chronological order, the earliest occurrences of the expression buyer’s remorse (also buyer remorse and buyers’ remorse) that I have found:

1-: From Woman testifies she fought back in ‘sharp’ car deal, published in the Daily News (Los Angeles, California, USA) of Wednesday 1^st May 1946 [page 2, column 6]:
—This text is an account of the preliminary hearing, on Tuesday 30^th April 1946, of G. Kenneth Vaughn and Richard E. Vaughn, who were accused of 43 counts of grand theft by trick and device, and criminal conspiracy; Mrs. Lee Millar Lowe said the following to the Municipal Judge:

Last Sept. 5, Mrs. Lowe said, she bought a 1941 Oldsmobile for a ceiling price of $1590, from the Vaughn lot in Inglewood. […]
After making a down payment of $855, she said, she signed two contracts; the first calling for $110 for 15 months, the second calling for $62 for the 14 months following the original payment.
When informed by the bank that all the payments were for $110, Mrs. Lowe said, she charged in on Richard E. and demanded an explanation.
Richard E. assertedly showed her “hundreds of other contracts” signed by other clients, and wound up by saying, “Mrs. Lowe, I believe you’re just suffering from buyer’s remorse.”
To which Mrs. Lowe said she retorted: “I have nothing more to say to a crook like you. You will hear from my attorney.”

2-: From the following advertisement for Farrow Nash Co., car dealers, published in the Ventura County Star-Free Press (Ventura, California, USA) of Thursday 18^th August 1949 [page 23, column 7]:

DON’T GET BUYERS REMORSE

SYMPTOMS

1—Deflated Pocketbook
2—Mental Depression Lasting for Months
3—Helpless Feeling Coupled with Knowledge That the Deal Is Made and You’re Stuck with It.
4—High Blood Pressure.

DO –

1—Buy Your Car From A Local New Car Dealer.
2—Buy Your Car From A Dealer Who Values a Satisfied Customer Above All Else.
3—Deal Where You Get Low Cost Bank Financing—No Hidden Charges.

3-: From A Beauty Bowls ’em Over, a short story by Jeanne D’Arcy, published in the Sunday Patriot-News (Harrisburg, Pennsylvania, USA) of Sunday 7^th October 1951 [Magazine: page 2, column 2]:

Like any woman with a bee in her bonnet, Helen wasn’t happy until she was buzzing around from store to store in search of a bowl that was a buy. Finally her feet gave out. She settled for a salad set that wasn’t quite what she wanted. Then buyer’s remorse set in.
“It wasn’t what I wanted,” she said, “so I decided I’d personally supervise the making of a salad bowl. A building contractor told me mahogany was the best bet for a bowl, and an old Italian craftsman said he could make one.”