A 30-year-old man was taken to hospital with serious injuries and died a short time later.
Shop owner Cecil Coley was taken to hospital with minor injuries and is being held on suspicion of murder.
Police also said a man, aged 19, presented himself at Manchester Royal Infirmary with stab wounds on Tuesday and was arrested on suspicion of robbery.
Both he and Mr Coley were expected to remain in hospital overnight and will be taken into police custody on Thursday.
'Hard worker' Police said a gun was found at the scene and was being forensically examined.
The dead man is believed to have been one of a number of men who tried to rob the shop.
Police said two people were in the shop "legitimately" at the time it was raided.
"On attendance, there were a number of people in and around the shop and a number of men had fled the scene."
He said it was not clear what the "full circumstances" were.
"We could speculate on what has gone on but that will come out in the investigation," he continued.
"Someone in the area has been in possession of a firearm. It's not clear where that firearm has come from."
'Real shock' Speaking at the scene, Mr Coley's nephew, Glen Simms, 49, described the shopkeeper as a "hard worker" and said he had owned the premises for 40 years.
Neighbour Mavis Ferguson, 71, said she saw an injured man "lying in the road".
"A lady from the community centre was trying to revive him but I knew he was dead," she said. "I could see the blood."
She also said she saw another man shouting: "Who has got a car? That's my brother."
Another resident, Lee Spiby, 30, said he tried to do CPR on the man who had been stabbed.
He added: "It's terrible. I am gutted I couldn't save him. There was nothing I could do. I tried my best."
Phil Rawlings, the Rector of nearby St Brides Church, said: "It's a real shock. It will have ripples right the way through the community but this place is really resilient."
The road remained cordoned off while police continued to investigate.