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".

The Derbyshire club, who play in the eighth-tier Northern Premier League Division One North, have lost all 19 league and cup games this season.
New Mills have conceded 68 goals while three managers have left since June.
"It's tough but we've got a new squad and the players are starting to gel," Millers boss Garry Brown told BBC Radio 5 live's Non League Football Show.
Former Norwich City midfielder Keith Briggs took over from Roy Soule, who stepped down in June, but resigned after just 23 days for a job with Sheffield United's academy.
Andy Fearn was put in charge in July and appointed former Manchester City striker Shaun Goater as his assistant.
But Fearn and Goater lasted just nine league and cup games before resigning after a 7-1 home defeat to Prescot Cables.
Brown, who has overseen 10 league and cup defeats, added: "There's been a lot of changes to the squad and there's only three players still here from when we took over in September.
"There's no budget, it's petrol money these lads are playing for."
All is not lost for the Millers, who are bottom of the table, 10 points behind Harrogate Railway Athletic, the next team above them.
Brown, along with Paul Williams (his assistant at New Mills) and Lee Gregory, last season led Manchester team Wythenshawe Town to an astonishing 39 wins from 39 games played.
Bashley, who play in the Southern League Division One South & West, are also without a point after losing all 14 league games this season.
But they did manage a win in the FA Trophy preliminary round.
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.
[NEXT_CONCEPT]
If Chelsea boss Jose Mourinho thought he was having a bad time, he should spare a thought for New Mills.