Sellars, who has rebounded from injuries to both feet, recorded 17 points, five rebounds, five assists, and one steal.
"He makes real good decisions with the basketball," Coatesville coach Rick Nelms said.
The Red Raiders won without 6-4 senior swingman Leroy Hoggard, the Ches-Mont League National Conference's most valuable player, and 6-5 junior reserve Tyler Burke.
Hoggard was ejected in Saturday's opening-round win against District 3's Lancaster McCaskey and, per PIAA rules, had to sit out vs. the Eagles. Burke has a right-foot injury.
"Without them, we had to stick together and do whatever it took to get the job done," 6-5 junior forward Jaquan Hollingshed said.
Sellars, who is bound for St. Peter's, shot 6 for 11 from the field and 5 for 7 from the line. After injuring his right foot in the fall, he suffered a stress fracture in his left foot in late January.
"I'm just starting to get my groove back," he said. "I'm healthy now, which makes a world of difference. I can do the things I'm used to doing."
In a quarterfinal Saturday at a neutral site and time to be determined, Coatesville (25-7) will face St. Joseph's Prep (23-5), which ousted District 3's York in second-round action.
Against Norristown, Hollingshed contributed 14 points, 13 rebounds (five on offense), and five blocks.
"He's not that big, if you look at him, but he plays big," Nelms said.
Late in the third quarter, with the score tied at 34, the Red Raiders took control with a 7-0 spurt consisting of Sellars' pull-up jumper, Hollingshed's inside bucket, and Sellars' three-point play.
Early in the fourth, Sellars found Hollingshed for an alley-oop slam that made it 47-37.
"That was my first one of the season," Hollingshed said. "I had missed a couple before that. I had to finish that one."
Senior lead guard Luke Kelley paced Norristown (24-5) with 15 points.
The Eagles' Josh Johnson, a 6-6 senior who grabbed 11 rebounds, was slowed in the second half by foul trouble.
Coatesville 16 10 15 17 – 58
Norristown 13 11 12 16 – 52
C: Jaquan Hollingshed 14, Andre Boggs 6, Emmett Hunt 13, Chris Jones 5, Howard Sellars 17, Joey Lewis 3.
N: Luke Kelley 15, Bernard Gordon 7, Josh Johnson 10, Marquis Bryant 8, Jahbri Hargrove 9, Curt Lever 3.
Contact Rick O'Brien at firstname.lastname@example.org. Follow @ozoneinq on Twitter. Read his blog, "The O'Zone," at www.philly.com/ozone