Well the lawyer represents the parents and according to his article that are claiming the bullets legally should not have been sold the buyers .
“Hampton’s family’s attorney Craig Heidemann said the family respects people’s right to bear arms but wants to encourage firearm dealers to obey the regulations in place to keep guns and ammo out of the wrong hands.”
“Just before 7 p.m. on Oct. 31, 2018, Perez, Burden and Aaron Anderson went to the Walmart on South Campbell Avenue in Springfield to buy the bullets, but the sales associate at the gun counter noticed something was off and refused to sell them bullets, according to a lawsuit filed earlier this month in Greene County.” “So, the lawsuit says, the trio proceeded down Campbell Avenue to Academy Sports and Outdoors, where Burden was able to purchase a box of 9mm ammo.”
https://www.news-leader.com/story/news/crime/2020/05/13/springfield-academy-sports-outdoors-family-sues-sell-bullets-homicide/3114651001/
Yes, the person who committed the murders was not allowed, legally to have firearms or ammunition.
The question is: should the clerk have, as a reasonable person, assumed that to be true?
Well presumably, at Walmart they acted in a way that tipped off the clerk, and he refused the sale. Either by chance or by having learned their lesson at Walmart, they managed not to tip off the clerk at Academy Sports and he completed the sale. The plaintiff acknowledges this was a straw purchase by someone legally allowed to buy, but claims the Academy clerk should have known that.