Notably, 94 percent of the ads were posted by “private parties,” who, unlike licensed dealers, are not required to conduct background checks.

Image Jitka Vesel was killed by Dmitry Smirnov, who was stalking her and bought a gun online.

Besides Mr. Roman-Martinez, the Times investigation led to Gerard Toolin, 46, of Walterboro, S.C., who is a fugitive from the Rhode Island police and has two outstanding felony warrants as well as a misdemeanor warrant. His legal status bars him from owning guns, but he was recently seeking to buy an AK-47 assault rifle on Armslist and was also trying to trade a Marlin rifle. He posted photos to his Facebook account of an AK-47 he had already purchased, along with a variety of other guns.

There was also Martin Fee, who has a domestic battery conviction in Florida and other arrests and convictions in Florida and New Jersey, including for drug possession, burglary and larceny. He was selling a Chinese SKS rifle on the classified section of another Web site, BudsGunShop.com.

The examination of Armslist raised questions about whether many sellers are essentially functioning as unlicensed firearms dealers, in contravention of federal law. The law says that people who “engage in the business” of selling firearms need to obtain a license and conduct background checks on customers. While the definition of engaging in business is vague, The Times found that more than two dozen people had posted more than 20 different guns for sale in a several-month span.

Among them was Joshua Lovejoy, 32, who since November has advertised more than 100 guns on Armslist, mostly in Canton, Ohio, ranging from AR-15 assault rifles to Glock 19 semiautomatic pistols. He once listed more than 20 guns in a single ad. He insisted in a telephone interview, however, that he had sold only a few.

Then there was Ron Metz, 49, who has advertised more than 80 guns from Anderson, S.C., since February. Mr. Metz said in an interview he had needed money, so he started selling some guns and trading for others. He also bought other guns, which he turned around and sold as well. He said he had no real idea how many he had sold, guessing that it was more than a dozen. He never keeps any records and does not do any background checks, explaining: “I can just sort of read people.”

“You think I broke a law?” he asked.

‘A Gun Show That Never Ends’

Armslist was the brainchild of Jonathan Gibbon and Brian Mancini, friends who attended the United States Air Force Academy and then transferred to the University of Pittsburgh.