The 32-year-old defender once sold water coolers to his teammates at Queens Park Rangers and also bought 30 iPads off the internet and then sold them to his fellow Nigeria internationals when they came to visit London.
"I have always been a wheeler-dealer," he told The Express. "I used to buy and sell everything.
"If you ask a lot of the players I have played with, especially the ones during my Charlton and QPR days, that's all I used to do. I used to enjoy playing football and doing business. You have got to make a profit. It's not a business if you don't make a profit.
"Football's my job, one I love doing, but I try to switch off from it as much as possible. Football is my life, but I do many other things as well. I have four kids, they take up most of my time, but I also love doing business."
Shittu will feature for Millwall on Saturday when they take on Wigan Athletic in the semi-finals of the FA Cup at Wembley.