By HIROKO TABUCHI and RACHEL ABRAMSJAN. 7, 2016

Jack Strahan has been a small-time trader of guns and gun parts for years, and he is not stopping now.

“It’s the way we do it down here,” said Mr. Strahan, 67, who has worked a number of odd jobs over the years in Hudson, Fla., and who says he sells, at most, a few guns a year. His current collection includes an AK-47, an AR-15, a Remington 742 and several Mossberg shotguns.

“You get a gun, you play with it for a while, then you trade or sell it, then get another one,” he said. “No one’s going to stop us doing that.”

Mr. Strahan is one in a large universe of gun enthusiasts who buy and sell firearms — online, at gun shows and even on Facebook — with no dealer’s license, no official records and no criminal background checks.

Federal Bureau of Investigation records show that nearly 21 million gun sales were processed through the background-check system in 2014, and retail sales of firearms came to an estimated $3.1 billion in 2015, according to IBISWorld, a research firm. But some industry analysts say as many as 40 percent more firearms might have been sold through private transactions not subject to background checks — a figure that gun advocates vigorously dispute.

On Tuesday, President Obama thrust enthusiasts like Mr. Strahan into the spotlight by clarifying that even irregular gun sellers could be considered dealers under federal law, and could face stiff penalties for selling firearms without a license.

Still, the gun industry, from hobbyists like Mr. Strahan to the National Rifle Association, has largely shrugged off Mr. Obama’s efforts. Experts said it was unclear whether the plan would push unlicensed dealers to stop operating on the margins of federal and state record-keeping systems.

Mr. Strahan said he might stop dealing openly on websites, like Armslist.com, which have come under recent scrutiny for hosting what appear to be unlicensed dealers. But he can always turn to more private trading clubs on Facebook, many of which operate in his area, to continue to trade firearms, he said.

“I don’t think you’re going to see scores of individuals racing to A.T.F. to be licensed as a dealer,” said Mike Sullivan, the former director of the Bureau of Alcohol, Tobacco, Firearms and Explosives, which issues federal licenses to gun dealers.

“If you are engaged in the business of selling firearms, and you know that, and you’re knowingly not pursuing a license, this is not going to make you go out and get a license,” Mr. Sullivan said.

Despite its limited scope, gun-control advocates have welcomed Mr. Obama’s actions as a way to crack down on a small segment of unlicensed dealers who are thought to sell dozens or even hundreds of firearms a year.

On Wednesday, a White House spokesman, Josh Earnest, said that clarifying the penalties for “hiding behind the hobbyist exemption” would spur a substantial number of sellers to comply with existing law.

A study of unlicensed sellers on Armslist.com, published in November by the gun-control advocacy group Everytown for Gun Safety, found that of the more than 600,000 firearms ads posted on the site in the year through mid-October, some 30,000 — or 4.3 percent — came from high-volume dealers who sold 25 to 150 guns a year.

“What this does suggest is that high-volume sellers make up an important but relatively small share of the gun sales without background checks that are currently occurring in the United States,” said Ted Alcorn, research director at Everytown and author of the study.

Going after this group, though small, gives law enforcement officials “more bang for their buck” in cutting down on unlicensed gun sales. But he said it was unclear how much Mr. Obama’s actions would affect the behavior of smaller sellers who made up the bulk of unlicensed gun sales.

Armslist did not respond to a request for comment.

Scott Schmoke, a chef in Daytona Beach, Fla., says he uses Facebook forums to post listings for the two to three guns he sells a year. For now, he has no plans to get a dealer’s license, though he said some of his friends were considering getting one after Tuesday’s announcement.

Mr. Schmoke, 53, said he was a responsible seller. He never ships guns through the mail and always meets buyers in person. Like Mr. Strahan, he asks to see a driver’s license, but a background check is not required.

“If I think they’re thugs or drug dealers, I use my own judgment and don’t sell,” Mr. Schmoke said.

Facebook did not immediately respond to a request for comment. But in 2014, the social network said it was taking steps to regulate gun sales on its site, including deleting posts that seek to circumvent gun laws, and restricting minors from viewing pages that sell guns.

People can still purchase guns legally online. But guns must be shipped to a licensed dealer who can perform a background check on the buyer.

Dan Hinkson, a licensed dealer in Fredericksburg, Va., says there is a misunderstanding about firearm purchases over the Internet. He sells guns through his website, Be Ready Tactical, and will ship only to other licensed dealers.

“Nobody is buying guns on the Internet legally right now without going through a background check,” he said.

Of course, the Internet is not the only source of unlicensed gun sales. Gun shows have long been the target of law enforcement because they are often heavily attended by enthusiasts, collectors and sellers.

“If you go to a place like Arizona or Texas, you’ve got individuals who go to gun shows, they advertise on the Internet, maybe they sell out of homes,” said William Vizzard, a former A.T.F. agent and professor emeritus at California State University in Sacramento. “I’ve seen as many as 250 guns on a table at a gun show, and often they don’t run any checks.”

There is no consensus about whether most sales at gun shows involve background checks.

Charlotte Jubinski, co-owner of the gun show organizer Mid Atlantic Arms Collectors, made them a requirement, and says she is vigilant.

Ms. Jubinski said she did not expect Mr. Obama’s actions to have much effect on turnout or sales at her gun shows, including one scheduled this weekend in Binghamton, N.Y., which will feature about 100 dealers of guns and other hunting gear.

“Every now and then, we’ve seen people out in the parking lot with the trunk open, trying to sell guns,” said Ms. Jubinski, 61.

“We call the police on them.”