Police said the victim had arranged to meet up with the strangers to sell them car parts about 11 p.m. Monday.
The group of at least 10 men in several cars ambushed the seller at the meet-up site and shot at him 11 times, hitting him twice, police said.
The victim was able enter the McDonald's from the parking lot and asked for help. At that point, a good Samaritan took him inside and provided first aid care before an ambulance arrived.
"It’s obvious the man knew what he was doing," Methuen Police Chief Joe Solomon said "He provided first aid, applied pressure, put clothing on the wounds. So we’d like to thank that individual for his heroic actions.”
Authorities said they have surveillance video and are working with another police department to find the men responsible for the shooting.