Lawsuit has potential to become test case that could have widespread ramifications for burgeoning internet trade in guns

The family of a woman who was murdered last year by a stalker wielding a .40-caliber handgun bought illegally on the internet is suing the gun website through which the firearm was procured for allegedly causing the shooting.

The legal action against Armslist.com, a web listing site that specialises in firearms, is the first of its kind and opens up a new front in the ongoing battle to tighten America's loose gun controls. It was filed in an Illinois court on Wednesday morning by the Brady Center to Prevent Gun Violence on behalf of the brothers and father of Jitka Vesel, a 36-year-old immigrant from the Czech Republic who was fatally shot on 13 April 2011.

The legal action will be closely watched by those involved in the booming internet trade in guns. At least 4,000 sites engage in weapons sales in the US, just 10 of whom list more than 25,000 firearms for purchase at any one time.

"Gun sellers and website operators who knowingly funnel guns to killers and criminals must be held accountable. We as a nation are better than an anonymous internet gun market where killers and criminals can easily get guns," said Jonathan Lowy of the Brady Center.

Under a loophole that has been widely denounced by gun control advocates, individual unlicensed dealers are classified as "private sellers" and as such are not obliged to carry out background checks on their customers. Private sellers ply their wares through gun shows and increasingly on the internet, fuelling the black market in illegal guns.

The killer of Jitka Vesel was a Russian immigrant living in Canada called Demetry Smirnov. He had a very brief relationship with Vesel but after she broke off contact a few years ago he began to stalk her. He tracked her in the Chicago area using a GPS device he glued to her car, then shot her 12 times as she was leaving work.

Smirnov had bought the Smith & Wesson murder weapon from a Seattle bartender who posted it on Armslist.com. The seller, Benedict Ladera, went ahead with the deal even though he knew that Smirnov was a foreigner and that it is illegal to sell guns to an out-of-state resident. Ladera added an extra $200 to the $400 sale price as compensation for the risk he was taking in committing a felony.

Smirnov pleaded guilty to the murder in July 2011 and will spend the rest of his life behind bars. Ladera was sentenced in June to a year in prison for having made the illegal sale.

At the time of Ladera's sentencing, a US district judge in Seattle remarked that "it's a sign of the times; guns are killing people all over our country. We are allowing too many people to have too many guns without any meaningful control".

The Armslist.com lawsuit has the potential to become a test case that could have widespread ramifications for the burgeoning internet trade in guns. Weapons sold online have been linked to some of the most devastating US shootings in recent years.

In 2007, Seung-Hui Cho carried out the Virginia Tech rampage, the deadliest mass shooting by a single gunman in US history. One of the weapons he used was a Walther P22 semi-automatic bought from the internet gun retailer TGSCOM. The following year the same website sold Glock magazines to a mass shooter who killed five students at Northern Illinois University.

Armslist.com is one of the largest internet gun showrooms in the US, with listings that currently run to 2,902 web pages, mainly from private sellers. It has also been named in connection with guns used in past shootings.

In October, Radcliffe Haughton shot and killed his wife and two others in a spa in Milwaukee, Wisconsin, having bought a gun through Armslist.com with no background check and despite the fact that he had a domestic violence restraining order against him. Gwen Moore, a congresswoman from Milwaukee, has written to Armslist, LLC, the owners of Armslist.com, urging the website to do more to prevent illegal gun sales. "We urge you to continue working towards preventing future senseless tragedies," she wrote.

Last year, New York City, a leading proponent of greater gun controls through the office of its mayor Michael Bloomberg, carried out an undercover investigation into the proliferation of internet gun sales. It found that 62% of private online sellers agreed to trade weapons to New York's investigators posing as buyers even though the purchasers openly admitted they probably couldn't pass a background check. It is illegal under federal law, even for private sellers, to sell a gun knowingly to someone who would not pass a background check.

In the case of Armslist.com, New York found that 54% of the private sellers they approached agreed to sell to an undercover buyer who admitted they were unlikely to pass a background check. The investigators posing as buyers actually met with private sellers operating through Armslist.com and bought four weapons including a Smith & Wesson handgun similar to that which killed Jitka Vesel.

A New York investigator recorded his conversation with one seller. "No background checks?" the investigator asked.

"No, I just take cash, and there you go," the seller replied.