Online gun sales have become a haven for buyers looking to avoid background checks, leading gun safety advocates concerned they are becoming more of a problem than sales at private gun shows, according to a new report by a progressive think tank, the Washington Post reported on Monday.

"At this point, this is the biggest loophole in the background check system," Third Way social policy and politics director Lanae Erickson Hatalsky told the Post following the release of the group's report, which found that among the advertisements for more than 15,000 guns on the sales site Armslist, in 10 states where lawmakers voted against bills that would have required background checks for private gun sales were nearly 2,000 listings by people looking to buy their firearms privately.

"Nobody's monitoring this," Hatalsky told the Post. "Nobody has any ability to stop these people who are looking for private sellers -- and the only reason to do that is to evade the background check system."

An April 2013 bill by Sens. Joe Manchin (D-WV) and Pat Toomey (R-PA) that would have required online sales to be subject to background checks did not win enough votes to pass, despite widespread public support.

Meanwhile, two Democratic lawmakers plan to introduce a bill that would protect victims in domestic violence situations from being vulnerable to a gun attack by their abusive partners, Think Progress reported on Monday.

The bill by Connecticut Democratic Sens. Dick Blumenthal and Chris Murphy would make it illegal for a person under a temporary restraining order to buy or possess firearms. Current federal law already prohibits people under permanent restraining orders from doing so, but in many jurisdictions, it can take up to two weeks before permanent orders are put into effect, meaning abusers can potentially gain access to weapons even under a temporary order.

"The maximum point of danger for anybody in a relationship is when that person says to the other that they are separating," Blumenthal told the Hartford Courant on Friday. Murphy also issued a challenge to Republican senators, who have tended to oppose gun safety legislation.

"If you don't want to step up and protect vulnerable women and domestic partners, who do you want to protect?" Murphy told the Courant. "If they can't support this, I think it's an admission, at some level, that they can't support anything."

A report (PDF) by the Connecticut Coalition Against Domestic Violence showed that 38 percent of the 175 deaths in the state linked to domestic violence between 2000 and 2011 were caused by firearms.

Third Way's report on online gun sales can be read below.

[Image: "Man With Gun" via Shutterstock]