“This fusion allows us to take pole position in Europe, with 2.5 million cars sold every year on our different platforms,” CarNext chief executive Sophie Krishnan told AFP.
Online marketplaces for used cars have grown during the Covid pandemic.
Constellation raised more than more than £1 billion ($1.4 billion, 1.2 billion euros) in May to develop the Cinch used-car platform.
The UK company also owns BCA, a business-to-business online marketplace for used vehicles.
Some 1.7 million cars are traded between dealers, manufacturers and leasing companies on the platform in 14 European countries every year.
The group also buys used cars from consumers in the UK via its WeBuyAnyCar platform.