Shahzad Ali Shah, 23, of Crawley, offered £5,500 for the Mercedes and, accompanied by his family, took the cash to east London.
They were approached by four men, one of whom produced a gun. Shah was then forced to hand over the money without seeing the car.
Police said the incident took place in Dagenham on August 20.
Shah was with his mother and brother-in-law when the robbery took place. A Metropolitan Police spokesman said: 'There were three victims, two men and a woman, who were buying a car advertised on an internet site.
'They were approached in their car by four men, three black and one Asian man.'
He said an Asian man produced a gun when the three victims refused to hand over the money.
Police have not confirmed how much money was stolen.
A spokesman for eBay said it was the first such incident involving website users that he was aware of.