Phillip Shortman, 27, from Abersychan, Pontypool, has admitted 10 counts of fraud and will be sentenced on Friday.
The charges related to selling a "non-existent" phone and vehicle parts on eBay and another website.
Newport Crown Court heard he has 17 previous convictions for 77 offences, many related to eBay.
In the latest case, Shortman admitted using his five-year-old son's details to sell a bogus Samsung Galaxy S4 mobile phone through the online market place.
He arranged for £480 to be transferred into his wife's account, but failed to deliver the phone.
Shortman then attempted the same con again, being paid £440 for the same phone.
The court was told he then "attached" himself to a website selling Suzuki Jeep parts, calling himself Yuppy Bear.
The Essex police officer who failed to receive the parts he ordered from Shortman helped fraud officers in Cardiff arrest him.
Jeffrey Jones, prosecuting, said the value of his fraud was estimated at £2,065.
Mr Jones told the court Shortman had arrived at court with the sum in cash.
The court heard one of his previous crimes, committed when he was 17, had 80 victims.
He has also been convicted of selling bogus Wales Rugby international tickets, worth £7,000.
Nigel Friar, defending, said much of Shortman's offending "had its basis in immaturity and a lack of understanding of money". He said: "Mr Shortman is finally beginning to grow up."
He told the judge, John Jenkins QC, Shortman was remorseful, and "custody has been imposed on a number of occasions since 2007", adding "some would say that has failed".
A prolific eBay conman has been caught again after failing to deliver a car part to a police officer who ordered it from him online.