Helge Achenbach, 62, who has been in pre-trial detention for about six months, denies the accusations against him, which carry a maximum jail term of 10 years.
The art consultant's contested deals included paintings by Pablo Picasso, Roy Lichtenstein and Gerhard Richter, while the vintage car sales were of Ferraris, Bentleys and Jaguars, Spiegel reported.
Prosecutors accuse Achenbach of more than 20 counts of fraud, as well as charges of forgery and breach of trust, in the trial in Essen, the court said in a statement.
The state charges that he defrauded the late Berthold Albrecht, heir to the Aldi Nord supermarket empire, of €22.5 million ($27.7 million) in the sales of 14 artworks and nine cars, by deceiving his client about the original purchase prices.
Achenbach is also accused of having defrauded two other wealthy customers in art deals, the court said.
In the dock with him is a former business partner, identified by the court only as Stefan H. in keeping with German reporting rules for criminal defendants.
In a separate civil case, before a court in the defendant's home city of Düsseldorf, the family of supermarket tycoon Albrecht, who died in 2012, is suing Achenbach for almost €20 million in damages.
The Essen court has set an initial eight further court hearings until January 21.
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