MADISON, Wis. — An online weapons dealer who sold a gun or accessories to three mass killers, including a man who opened fire at a Pittsburgh-area health club this week, said Friday that any of the shooters could have just as easily found what they wanted at a Wal-Mart or another store.

Eric Thompson, whose company TGSCOM last year sold an empty Glock 9 mm magazine and magazine loading apparatus to George Sodini, the man who shot up a Collier Township health club on Tuesday, said the sale was legal and his company did nothing wrong.

"The firearms industry and firearms dealers are lambasted by the media and by politicians all the time and very often nobody stands up and says 'hey, we didn't do anything wrong,"' Thompson said. "I'm ... being penalized by doing a good job and employing a lot of people and selling sporting goods ... I'm not some backwoods guy just making to look a buck off of tragedy."

Thompson's company, which is based in Green Bay and employs about 40 people, also sold a gun or accessories to the shooters in the Virginia Tech and Northern Illinois University attacks.

Seung-Hui Cho used a .22-caliber handgun bought from TGSCOM in his attack at Virginia Tech in April 2007, in which 32 people were killed. Stephen Kazmierczak, who killed five people in an NIU classroom before killing himself in February 2008, bought two empty 9 mm Glock magazines and a Glock holster through a TGSCOM site.

Andrew Goddard, whose son Colin was shot and wounded at Virginia Tech, said Thompson "needs to have some sort of social responsibility."

"I take issue on the basic principle that he's selling something that doesn't have a legitimate use outside of stalking and killing people," Goddard, of Richmond, Va., said.

It was not immediately clear if Sodini used the TGSCOM-bought accessories during his attack. Sodini, 48, fatally shot himself after opening fire on a weekly Latin dance aerobics class.

Federal law prevents the public from purchasing a handgun in any state but their own. People can purchase long guns, such as rifles, in border states. Firearms dealers, however, can transfer weapons to one another across state lines.

Online dealers often offer a larger selection of guns than their brick-and-mortar counterparts. Someone who wants a gun from an online site must first find a licensed firearms dealer in their home state who will receive it and conduct the proper background checks.