The Toffees boss re-signed the South African from Tottenham for £4.5million following a successful loan spell - just 18 months after selling him to the north London club for £3million.
Moyes has made huge profits in the past by selling some his prized assets such as the sale of defender Joleon Lescott to Manchester City, which earned Goodison Park club a mammoth £17million.
But the Scot has revealed his biggest gamble was bringing 30-year-old playmaker Pienaar back to Merseyside.
He told The Sun: "At Everton we have always tried to do good deals and have always tried to buy at the right age and the right price.
"We had to buy Steven back for more than we sold him for. That didn't sit well with me because I didn't particularly see it as a good business deal.
"But I knew on the football side of things he would make us better. It's been a great deal for us and we're delighted to have him."
Moyes added: "Steven wanted to come back - that was the biggest thing.
"He wanted to come back more than anything and because of that it swayed me to do the deal."
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