The study compares properties listed on Zoopla by Purplebricks, eMoov, Tepilo, HouseSimple and House Network.
The comparison site says these five agents shared approximately 75% of the 15,000 properties listed for sale (excluding those under offer/sold subject to contract) by online estate agents on Zoopla at the start of May.
As of the beginning of this month, Purplebricks had 6,582 properties listed for sale on Zoopla.
This is more than the other four agents combined (4,495) and significantly ahead of second placed eMoov at 1,392.
The figures also show that Purplebricks' stock levels have increased significantly since its flotation last year.
At the start of December – before listing on the London Stock Exchange's Alternative Investment Market – Purplebricks had 4,194 properties listed for sale on Zoopla.
The figures show that the agency's stock levels have increased each month since December.
The other four agents' stock levels dipped in January before improving in February.
SellingUp's research paints a similar picture to some statistics released by Rightmove at last week's FUTURE: PropTech conference.
The portal's survey of thousands of home movers found that the agency run by Kenny and Michael Bruce was ahead of its online rivals when it comes to consumer awareness.
Purplebricks was 'known' by 76% of participants when prompted and 19% of these people said they would consider using them to sell their home.
The next highest figures were for Sarah Beeny's online agency, Tepilo, which scored 39% for consumer awareness, with 9% of these participants indicating they would consider using the firm.
“Purplebricks has clearly benefited enormously from the additional funds it has been able to access since becoming a public company in December," says Oliver Lewis, SellingUp's research director.
"Its growth is likely to have been driven by substantial advertising spend which has enabled it to pull further away from its rivals to become an apparently unassailable market leader."
“It is hard to see another online estate agent seriously rivalling Purplebricks for market dominance in the near future without a merger taking place or a new flotation that might bring in enough shareholder money to fund a challenge,” he says.
Purplebricks' pre-float prospectus showed the company made significant losses in its first three years of operation.
In its trading update to the City this month, the agency reported that it expects full year revenue to be approximately £18.5 million.
It also stated that it intends to move into profit in 2017.
Purplebricks' next update will see it announce its full year final results on June 16.