The mystery seller bought the medal after the former Hoops captain donated it to an auction at a Jim Baxter testimonial dinner several years ago.
The Celtic giant – affectionately known as Cesar to fans – was presented with the medal at the end of the 1973-74 season when the Old Firm club became the first to win the Scottish League nine consecutive times.
It's being put up for sale in Edinburgh next Monday by Convery Auctions. The firm's owner, self-confessed Celtic fan David Convery, said: "You would normally expects bids of between £1500-£2500, but given it's the captain's medal and Celtic were the first to achieve nine-in-a-row, bids could be as high as £3500.
"There's already been a lot of interest from the Glasgow area."
The McNeill medal isn't the only silverware up for grabs at the auction.
A 1897 Scottish Cup medal belonging to the late Rangers striker John McPherson is also being sold for between £1000-£1500.
Another lot of interest to Old Firm fans will be a 1892 Glasgow Charity Cup medal won by Celtic great Alex "Duke" McMahon, which is expected to fetch between £1500-£2500.
Former Partick Thistle and Scotland goalkeeper Alan Rough's debut international cap is also up for auction after it too was snapped up several years ago when he sold some of his footballing memorabilia.
The keeper was presented with the cap on April 7, 1976, at Hampden Park when Scotland beat Switzerland 1-0. It is expected to generate bids of between £400-£600.
More than 400 items are to be auctioned next Tuesday at the Royal Highland Centre at Ingliston, with football, cricket, golf, rugby and boxing memorabilia all going under the hammer. These will include a selection of rarely seen Scotland football shirts, which were designed for but never worn by the team.