But the "Whixall Monolith", discovered by James and Jasmine Dowley, while excavating a driveway, could yet remain in the county as negotiations take place with the Shropshire Museums Service.
The artwork, carved onto a large Permio-Triassic red sandstone block, was discovered in July last year at Whixall, near Whitchurch.
Timeline Auctions, which handled the sale, described it as "the oldest piece of art ever discovered in Shropshire".
At the time of its discovery, the sculpture was appraised by Peter Reavill from the Portable Antiquities Scheme and local archaeologist George Nash.
They said the distinctive cup-and-ring decorations suggests it may be linked to a burial chamber or other sacred site.
It was described as being in a 'fine but weathered condition', and was thought to be potentially of regional or even national importance.
Mr Nash said the markings were pecked 'cup-and-ring' and other marks, including the main element comprising a central circular 'cup' enclosed within four concentric pecked rings extending outwards.
He said: "Megaliths and other stones that have been enigmatically decorated with pecked cup-and-ring and other marks are well known in the British Isles and in Ireland, as well as in some areas of mainland Europe.
"They appear where the local geology provides a source of material or where erratic boulders, left behind when the glaciers retreated at the ending of the last Ice Age, are found."
Mr Nash said examples had been discovered in Northumberland, Yorkshire, Derbyshire and Cumbria, but this appeared to be the first example ever to have been found in the Shropshire area.
"The purpose or meaning of these markings remains uncertain but it has been suggested that they could have been placed on boundary markers, on stones used in chamber tombs or have been connected with sacred sites," he said.
"The design of a central cup depression surrounded by four concentric pecked rings appears quite frequently, at sites spread across Britain and northern Europe; some are on pillars or standing stones, others on very large in-situ boulders; many show very considerable weathering from being exposed on the surface for millennia."
Mr Nash noted that the base appeared to be less eroded and patinated than the other surfaces, so it could have been the apex of a standing stone monolith.
"The closest parallel to the Whixall Stone is seen with the Llwydiarth Esgob Stone, now sited away from its original context, in a private garden," he added.
Brett Hammond, managing director of Timeline Auctions, said the stone was sold to a Californian buyer for a total of £15,240, including the buyer's premium.
"Negotiations are underway to allow Shropshire Museums to have the piece on long-term loan so they can display it to the public," he said.