Last month, eBay filed a mysterious lawsuit against Craigslist saying that the listings company "unfairly diluted" its investment in it.

No further details were given.

But now, the case has got even curiouser with Craigslist countersuing and accusing eBay of actually embedding spies in its board.

The actual complaint reads that eBay is guilty of "unlawful and unfair competition, misappropriation of proprietary information, deceptive passing-off, business interference, false advertising, phishing attacks, free-riding, trademark infringement, trademark dilution, and breaches of fiduciary duty".

In particular, its says that eBay ran ads for rival business Kijiji on Google but made them look like Craigslist ads.

And, more than this, actually used its position as a shareholder in Craigslist to plant spies in the listings company, before launching Kijiji.

Reading through the legal mumbo-jumbo, the implication is a massive court case.

eBay wants Craigslist found guilty of disadvantaging its 28.4% stake; while Craigslist is asking the court to force eBay to say how much profit it has made as a result of its espionage; and "restore to Craigslist all shares of the company acquired by means of, or for the purpose of unfair competition" as well as "pay punitive damages for their malicious behavior".

eBay spokeswoman Kim Rubey responded to Reuters: "We regret that Craigslist felt compelled to resort to unfounded and unsubstantiated claims in order to divert attention from actions by Craigslist's board that unfairly diluted our minority interest".