Think your wife may be cheating on you? Wondering who your boss might be talking to? "Learn the truth. Spy today."

So reads an ad for "Bluetooth Spy Pro-Edition," one of nearly 200 mobile phone spyware products currently listed for sale on eBay.

The software, which costs as little as $3.99, can be used to view photographs, messages, and files on the phone, listen into phone conversations, and even make calls from the phone being spied upon.

Security experts are concerned because while these products aren't illegal, installing them without authorization to spy on someone else most definitely is.

And that's exactly what some of these products seem to be advertising. "You can now easily find out who your partner, business associates, friends have been in contact with," reads the Bluetooth Spy ad. "Whether you are suspicious of an affair or would just like information that will help progress your career, you can now do all of the following using your mobile phone, and the person you are targeting will not suspect a thing. Guaranteed!"

Another spellcheck-free ad claims that "You will now be able to establish who your freinds associates and husband/wife have been conversating with, you can read messages, even download them to your own phone or laptop, view their information and pictures."

This type of mobile spy software has been available for several years now, sold by companies like Flexispy and Neo-Call. Typically, however, it is much more expensive, and these companies are generally careful to promote only their legal uses such as monitoring corporate equipment, said Mikko Hyppönen, CTO with F-Secure. But the software is often used for nefarious purposes, such as industrial espionage and spying, Hyppönen said.

According to him, eBay shouldn't be selling this software; it is simply too dangerous.

Another security expert said that this type of software may even be harmful to the buyer. "You're certainly at a higher risk with the software of there being additional functionality that is not advertised and potentially malicious," said Craig Schmugar, virus research manager at McAfee's AVERT labs. "In general, when you see the advertising claims made and the types of pages represented, you should approach them with some skepticism."

This software can be installed via a Bluetooth connection and typically runs on both Windows Mobile and Symbian operating systems, McAfee said.

eBay representatives could not immediately be reached for comment on this story.