The rest of the name derives from the 293 bones in its skeleton.
Koller thought a private individual would be the likely buyer, although he was sure the public would still get to see it in future.
Discoveries of T-Rex fossils are extremely rare, said Hans Jacob-Siber, a paleontologist at the Aathal Dinosaur Museum in Switzerland.
"It's not a cast or a copy, it's the original. And there are very few, very few," Siber told Reuters. "In fact, until about 1970 or 1980, there used to be less than a dozen Tyrannosaurus, most of them were already in United States' museums."
Almost all of the other fossilised T-Rexes are housed in museums, meaning massive interest whenever a skeleton comes up for sale.
Two other T-Rexes discovered in North America - called Sue and Stan - fetched $8.4 million and $31.8 million respectively when they were sold in 1997 and 2020.
