Sanquhar Post Office in Sanquhar, Dumfries and Galloway opened its doors in 1712, and is recognised by the Guinness World Records as the first post office in the world.
It has become an unlikely tourist attraction for stamp collectors, as well as being a 'lifeline' for elderly people in the community who depend on it for banking.
But the accolade could be lost if the 307-year-old institution closes, as its owners plan to retire, and the title could be conceded to a branch in Stockholm, Sweden, which opened in 1720.
The Post Office bounced back from the brink of closure in 2015, when it was taken over by Dr Manzoor Alam, 77, and his wife Nazra, 67.
But the couple are now planning to retire, and are hopeful a new owner could be found to keep it open when they step down at the end of May 2019.
Nazra said: "It's breaking my heart but I've got to do what I've got to do.
"If my health was better I'd carry on but I've got to call it a day now - it's really sad.
"It served the community well.
"It's up for sale and I've put in my notice with the Post Office.
"At the end of May should be my last working day.
"What happens between now and then we'll just take it as it comes.
"It'd be nice if we could find a buyer.
"We're doing it because my husband's and my health isn't great.
"Next year it'll be five years since we've had it.
"I'd hate to think it'll shut down and become a supermarket or something.
"The last four years was remarkable, every bit of it."
The branch is on the market for £275,000, and comes with a three bedroom cottage.
The Post Office is popular with its banking service as Sanquhar, which is home to around just 2,000 people, only has one bank - after the closure of the town's RBS in 2014.
The branch is also an attraction for tourists, post enthusiasts and stamp collectors, who seek to have their letters hand stamped at the world's oldest Post Office.
Mum-of-two Nazra formerly worked as a nurse before taking over the branch with two part-time assistants.
But she now wants to live closer to her family in Birmingham.
Nazra said: "We've got a lot of banking taking place in the Post Office.
"We've only got one other bank, the Bank of Scotland.
"We get a lot of tourists during the summer as well."
Chairman of Sanquhar and District Community Council, Duncan Close, said: "We were been in this situation five years ago.
"It changed hands and was reasonably secure for a bit.
"We called a public meeting to discuss the matter and see what we can do.
"We will do everything we can to ensure the place does find a buyer.
"We're sorry to see them go but it's sad at the same time to think we could lose our Post Office."
Andrew Wood, SNP councillor for Dumfries and Galloway, said: "It'll be a sad day if it does close.
"What we're finding in a lot of rural places like Sanquhar is they're struggling to keep their post offices open.
"They're a lifeline for the elderly.
"It's not just about collecting their pensions, it's about meeting other people.
"I was wondering if the community could arrange a buy out but it shouldn't come down to that.
"We're a wealthy country and should be able to have them in good sized towns.
"I don't know why they've went down the road of being in shops - we've got to stop this centralisation."