ST. PETERSBURG — A young couple with a newborn child set out to buy a Samsung Galaxy smartphone in July. Instead, they ended up robbed at gunpoint, according to the Pinellas County Sheriff’s Office.This week, two men were arrested in that incident: Jermaine Brown, 20, and Robert Walker, 18, each face a charge of robbery with a deadly weapon.Brown was also arrested on a charge of attempted murder in a shooting that took place earlier this week, according to St. Petersburg police.The robbery took place on July 16 outside a home along the 5000 block of Yellow Pine Street at about 8:52 p.m., deputies said. The couple, both 23, attempted to buy a Samsung phone using the Facebook Marketplace app. They negotiated a price with the seller, who used the online alias "Destiny Dawes."But while parked outside the agreed-upon location, deputies said, a dark-colored vehicle pulled up and two armed men approached the couple with hoodies concealing their faces. They robbed the couple of $500 and other personal items, deputies said, and leveled a gun at the woman and her baby.No one was injured in the robbery. A partial tag number led detectives to Brown and Walker.Brown turned himself in to St. Petersburg police on Tuesday, where he was arrested on the attempted murder charge. Brown is accused of shooting a relative in the upper body on Sept. 2. The shooting victim identified Brown while he was being taken to the hospital, police said. When Brown was questioned about the July robbery, deputies said he admitted to the crime.Walker was arrested Friday at Boca Ciega High School in Gulfport. The Sheriff’s Office said Walker also admitted to robbing the couple.Both were being held late Friday in the Pinellas County jail.