After more than 100 hours of coding over three months, Mike Soule was finally ready to switch on his project. He didn’t know what to expect. If things went right, he could be on his way to financial success. If things went wrong, he could lose his savings.

His creation wasn’t a new mobile app or e-commerce store. It was a computer program that would buy and sell currencies 24 hours a day, five days a week.

DIY’s...