The 38-year-old from Worthing is currently in custody awaiting questioning after he was detained by officers.
Mr Manning, 54, of Brighton Road, was last seen on April 19 2014 before his family reported him missing on 22 April.
They said it was completely out of character for him to disappear.
Officers launched a murder investigation last June. A year on and despite extensive proof of life enquiries and searches, no one has reported hearing from or seeing him. His body has not been found.
A number of police vehicles attended Becket Motors, in Littlehampton Road this morning and a tent was set up on the forecourt at around 10.30am.
An eyewitness reported police driving a car parked on the forecourt away from the scene.
A police spokesman said: “The police activity at Becket Motors in Littlehampton Road, Worthing is one line of enquiry related to the Mark Manning murder investigation following the arrest.”
The arrest follows last week’s search of Brooklands lake, after police received a tip-off suggesting Mr Manning’s body could be found there. Click here for the coverage.
For a timeline of how the investigation has unfolded, click here
Anyone with information about the whereabouts of Mr Manning should ring police on 101 or email [email protected] quoting Operation Light or call the independent charity Crimestoppers anonymously on 0800 555 111.