After years of copycat recipes and attempts to guess the ingredients, McDonald's is selling a bottle of its "Big Mac special sauce" for the very first time – on eBay.

Only 200 of the 500ml limited edition bottles are being sold worldwide, with proceeds going to charity.

On Tuesday morning, "Bottle #1" had a bid price on eBay of 23,100 Australian dollars ($17,702), with eight days left of the auction.

Some lucky fans will also be able to buy small, 25ml containers of the sauce at 950 locations across Australia for 50 cents apiece.

For now the sauce is only available in Australia, and McDonald's could not be reached for comment as to whether this would be rolled out across the globe and when the next limited edition bottle will go on sale.

The eBay listing jokes that the Big Mac sauce will make "your boyfriend's gross lasagne" and "brussel sprouts" taste better.

McDonald's has been struggling with falling sales, and in the fourth quarter of 2014 reported net income of $1.1 billion, a drop from $1.40 billion a year earlier. Chief Executive Don Thompson will retire on March 1, and Steve Easterbrook, the restaurant chain's senior vice president and chief brand officer, will take the company's top position.

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Proceeds from the eBay auction will go to the Ronald McDonald House Charities which helps seriously ill children and their families.

The latest marketing stunt comes after McDonald's launched a campaign that allows some customers to pay for their food with "lovin"—such as fist bumping, hugging, or blowing a kiss.

Customers ordering food at a U.S. McDonald's will be selected at random and told they can pay in this way only after they have ordered, with the promotion lasting until Feb. 14.