Currently the checks are only required for sales handled through licensed gun dealers.
Sens. Joe Manchin, D-W.Va., and Patrick Toomey, R-Pa., said their accord would help keep firearms from criminals and the mentally ill.
The agreement could build support for President Barack Obama's drive to curb firearms violence. If the deal between two of the most conservative members of each party holds, it's believed that the bill will pass the Senate, which votes on Thursday.
"Truly the events at Newtown changed us all," Manchin said. "Americans on both sides of the debate can and must find common ground."
Background checks have gotten the most support from the public of all other gun control measures. According to a Quinnipiac University poll, 91 percent of respondents are in favor of requiring background checks for all gun buyers.
But the National Rifle Association remained critical of the deal.
"Expanding background checks at gun shows will not prevent the next shooting, will not solve violent crime and will not keep our kids safe in schools," the NRA said in a statement, adding that lawmakers should instead focus on fixing the country's mental health system and on gang violence.
The Associated Press contributed to this report.
