Officials from Dorset Trading Standards made a test purchase of a Manchester United shirt supposedly signed by Rooney for £150 after receiving complaints about Rennie's online business, FA Premier Signings.
They contacted Terry Baker, of A1 Sporting Memorabilia, which represents Rooney, and he took the fake shirt to the then Manchester United player.
From there Trading Standards launched an eight month investigation into Rennie and they established he had conned 4,500 victims over the years.
They included a mother who paid paid £300 for a 'signed' Thierry Henry shirt to cheer up her son after his father died and a woman who paid £280 for a football signed by 23 Liverpool players for her husband's 40th birthday.
Rennie, 46, from Banbury, Oxfordshire, was today found guilty of fraud charges following a four day trial at Bournemouth Crown Court.
He was warned by a judge that he faces a substantial prison sentence.
Rennie's estranged wife Clare, 45, had previously pleaded guilty to her part in the con and is also awaiting sentence.
Bournemouth Crown Court heard that Rennie sold 200 items supposedly signed by Lionel Messi, 272 'signed' by Real Madrid star Ronaldo, 335 bearing Liverpool's Steven Gerrard's signature and 220 with Wayne Rooney's name.
He also sold 'retro' shirts such as an old Brazil top bearing Pele's 'signature.'
Rennie supplied with his items a 'certificate of authenticity' but the prosecution say this was also fake.
The range of signed goods he offered for sale was 'staggering' and was also his undoing.
One item he sold was a 2013/14 Liverpool FC shirt bearing 24 autographs including Gerrard and Luis Suarez.
A brand manager from Liverpool gave evidence to say it was unheard of to get so many players to sign one item and that he would be delighted to get 14.
When officers raided Rennie's home they found a stack of unsigned football shirts and balls and boots, as well as the stamp for the certificates, blank certificates and permanent markers.
Rennie didn't pay tax on the money he made and spent some of it on family holidays to Florida, on once occasion taking £10,000 spending money with him.
Neil Martin, the principal Trading Standards officer who led the investigation, said: 'This was a deliberate and long-term fraud that conned 4,500 people into buying fake signed football memorabilia.
'To gather evidence in this case we have had to contact a number of FA Premier Signings' customers to tell them the items they purchased are fake and as such are worthless.
'This is after they have spent upwards of £150 for a single signed shirt to over £700 for a squad signed shirt.
'A successful investigation like this can only happen with the backing of evidence from those affected. This includes consumers, legitimate businesses involved and in this case Wayne Rooney who initially confirmed that a signed shirt we purchased was a fake.
'There are a number of genuine memorabilia business that spend a lot of time and money ensuring that the items they supply are genuine. We are very grateful to some of these companies who helped us with this case.'
Ivan Hancock, manager of Dorset Trading Standards, added: 'Eleven consumers provided evidence of their purchases for the case but clearly most purchasers will be unaware of being conned.
'Our first test purchase of a signed Wayne Rooney shirt was in February 2015 and Wayne Rooney was shown it the following month.
'A warrant was obtained and Rennies' house visited and goods and records seized in January 2016.'
Rennie claimed that he bought the signed goods via eBay and trade fairs and that he had also been conned by them.
He said he didn't know the gear he sold on to customers was fake.
Rennie, who is now a postman for Royal Mail, pleaded not guilty to running a fraudulent business over a period of about nine years and transferring criminal proceeds to his bank account.
He will return to court next month for sentencing.
Judge Peter Crabtree told him: 'You have been convicted of extremely serious offences, the sentence is inevitably going to be a custodial one.
'I am prepared to give you bail to sort things out while you are at liberty.'
