When Gary George bought a truckload of videotapes for $218 from a US government surplus auction more than 40 years ago, he planned to sell them to television stations – to record over.

Fortunately, he decided to hold on to the three tapes labelled “Apollo 11 EVA”, which have since been identified as the only surviving original recording of the first moon landing, in 1969.

Now the tapes – which include Neil Armstrong’s famous words “That’s one small step for man, one giant leap for mankind” – are to go on sale in July at Sotheby’s in New York, where they are expected to fetch as much as $2m.

The collection also includes footage of Buzz Aldrin walking in minimal lunar gravity, planting the US flag on the moon, collecting samples. It also shows the crew’s call with Richard Nixon.

George, now 65, was astonished. “I had no idea there was anything of value on them,” the retired mechanical engineer from Las Vegas told Reuters. “I was selling them to TV stations just to record over.”

George thinks he sold eight reels to television stations for $50 each. It was his father who spotted the three Nasa labelled reels.

“He was really into the space program and he said, ‘I think I’d hang on to those. They might be valuable someday. So, for that very reason, I pulled them out and hauled them around the country for the next 43 years. That’s how come they survived.”

In 2006, Nasa admitted that they could not find the original video recordings of the landing. It was not until two years later, when George was on holiday with a friend who worked at Nasa, that he found out they were looking for them.

“Quite frankly, I was sitting at the table drinking a beer and I said, ‘Well, damn, I have those,’” said George.

He did not have the equipment to watch the tapes, but after he started speaking to Nasa about handing them over he was able to see the footage for the first time in a special studio in California.