Customers will initially be able to buy the entry-level A140 Classic with W-plate and about 15,000 miles on the clock. They will be priced around £9,995, including delivery to the customer's address.
The company insists online prices will not undercut its official used car retailers.
From September the cars will be advertised on the website for the manufacturers' used car retailing operation, Mercedes-Benz Direct, together with finance and insurance offers.
The cars will be sourced from the Mercedes-Benz Direct centres in Hemel, Croydon, Bristol, Chelmsford, Poole and Waltham Cross.
A DC-UK spokesman said: "This is a revolutionary step since we are the first manufacturer to sell officially approved used cars via the internet."