HM Revenue & Customs wants to target businesses that have failed to register for tax, and individuals who fail to declare the money they make online.
It said this "hidden economy" could equate to Â£5.9bn a year in tax.
HMRC has launched a consultation on extending its powers to collect extra data from firms and individuals.
No specific firms have been named, but sellers on internet marketplaces such as online advert site Gumtree, holiday home rental site Airbnb and e-commerce giant eBay could be among those targeted.
In the consultation document published online, HMRC said data played a key role in enabling it to detect those not paying tax.
Existing laws allow HMRC to access certain data but it wants to broaden its scope.
"Data can be particularly powerful when it is collected from third parties who facilitate trade, either between businesses, or between businesses and consumers," the consultation document said.
"This is because they can provide information in bulk about the activity of large numbers of traders, and because third party data can be used as an independent check against the data that taxpayers themselves report to HMRC."
The document said the extended powers would affect businesses acting as intermediaries or providing electronic payment services.
HMRC stressed it would not target individuals who sold personal possessions, only businesses that failed to pay tax owed.
It said it would not seek to find out what people bought online, but rather what was sold and not declared.
"Effective tackling of the hidden economy will ensure a level playing field between those businesses and individuals who comply with their tax obligations and those that do not," the consultation said.
It said those who were tax compliant "should see little or no impact".
The 12-week consultation runs until 14 October.

Sylvia Blackburn, 72, suffered serious head injuries outside Bedford Road Lower School, Kempston, last December.
Brian Wright, 55, of Kempston, told Luton Crown Court he had been blinded by the setting sun.
He accepted he had struck Mrs Blackburn but denies causing serious injury by dangerous driving.
Mrs Blackburn is now being treated in residential care.
Judge Michael Kay QC paused the case for five minutes so Mr Wright could compose himself after he told the court how he had turned his victim's "world upside down".
Mr Wright said: "I understand Sylvia was a much-loved lollipop lady, who was loved by the children.
"I am truly sorry for what I have done to her and her family."
Mr Wright's VW Golf failed to stop for a red light, the jury heard.
The married mother-of-three, a lollipop lady for 20 years, had been on duty in Bedford Road as pupils left school.
When the traffic lights turned red, Mrs Blackburn, who was wearing appropriate clothing, stepped out to escort a woman and her six-year-old daughter across the road.
Prosecutor Sally Mealing-McLeod said traffic in the other direction had stopped, but the defendant did not, and hit her.
Mr Wright, of The Dell, said he could not remember if the traffic lights were red or green.
"I was totally blinded by the sun," he said. "I just heard a bump. I was in total shock.
"Since that day I have thought of nothing else other than Sylvia Blackburn and her family and the position I have put them in."
The court heard the sun was 10 minutes from setting and could have been shining into Mr Wright's eyes.
The trial continues.
Internet companies may have to provide more information on people and businesses who sell goods and services online, in a crackdown on tax evasion.
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A driver who knocked down a lollipop lady broke down in tears in court as he said he was "truly sorry" for her and her family.