Investigators found the mother, whose full name was not given, used the money to buy an iPhone, high-end sports shoes and other products, also online, it said.
Apple's products are wildly popular in China, where a teenager sold his kidney and used the funds to buy an iPhone and iPad in an incident widely reported last year.
But the couple told police that they wanted the girl to have a better upbringing than they could afford, since they already had two children.
"Giving away the child was not for obtaining benefits, but giving the child better guarantees," one said.
Shanghai police and prosecutors could not be immediately reached for comment.
Shanghai media reports did not give the amount the couple received, but their online postings referred to 30,000 yuan and 50,000 yuan ($4,900 and $8,200).
Apple last month launched the iPhone 5s, including a gold-coloured model, and the more budget-conscious iPhone 5c in China.