Woolworths animal cards: desperate parents pay $100 Latest craze: Calvin and Alex Rule with their collection of the popular Aussie Animal Cards. The bottom-dwelling pyjama nudibranch is finally enjoying its moment in the sun. The little-loved striped sea slug has the good fortune of featuring in an "Aussie Animals" card collection that is the biggest craze to captivate children (since the last craze, at least). It's no mean feat for a mollusc. Nor for the unassuming spotted cuscus, long-nosed potoroo and quokka - all part of the popular new Woolworths supermarket promotion. The trading cards are ostensibly free. But desperate parents are paying more than $100 online to complete their child's collection. Sought-after cards being horse-traded in schoolyards, supermarkets and online. Packs of the free cards are given to Woolworths customers for every $20 purchase. But parents and grandparents have paid well over $100 for complete 108-card collections on eBay and Gumtree. Individual free cards, such as the wandering whistling-duck, whose natural habitat is swamps and sewerage farms, are selling for more than $2 each. Empty card albums, which have sold out in supermarkets, are also being scalped online. One eBay seller has reaped more than $3000 in four days. "I am absolutely stunned what people are paying," he said. The scalper bought 50 of the $5 albums last Thursday and has been re-selling them online for $70. I think it's pressure from their children. I am exploiting the laziness of parents who couldn't be bothered buying them earlier," he said. One person emailed asking him to donate a card album for their autistic son, he said. I later recognised that same email address selling the albums on Gumtree. That's how crazy this whole thing has got. Woolworths has announced it will reprint the collectors' albums to meet demand. Millions of free cards have been handed out to shoppers, a spokeswoman said. The promotion, which Woolworths ran in partnership with Taronga Zoo, ends on Tuesday. Consumer psychologist Adam Ferrier said parents were paying to fulfil the wishes of their children. Because the cards are innocuous and have some kind of educational value, the parent can rationalise indulging their child. Leah Rule of Mortdale has been helping her sons Calvin, 7, and Alex, 5, swap free cards on Facebook. "Alex really loves the animals and Calvin likes collecting," she said. Calvin added a clownfish at a school swap-meet on Monday. He is still short of a koala and luna lionfish. But Mrs Rule won't pay for them. "I think missing a few here and there doesn't matter," she said. After a couple of weeks they'll be on to the next big thing anyway.