Barbara Lapointe, a mother of two who operates a dog walking business, wants Kijiji to instead promote adoption from registered animal rescue groups and individual families.
Lapointe's online petition, posted on the website Change.org, had collected nearly 48,000 signatures by midday Sunday.
"We want them to set the example and be the ethical leader on this," Lapointe said in an interview.
Lapointe is also trying to get her local municipality, the Town of Mount Royal, to ban the sale of animals in pet stores.
She's hoping it will follow the lead of other Canadian cities such as Richmond, B.C., which banned the sale of puppies in pet stores in 2011.
Lapointe said cracking down on pet stores would eliminate a major marketplace for puppy mills but even then illegal breeders would "still be able to resort to online sales."
Kijiji Canada has no plans to change its policy.
Shawn McIntyre, the site's community manager, said it has set up a system to catch suspicious posts and prevent mills from using the site.
"We're choosing to work with the industry and do our best to keep it clean and safe," McIntyre said, explaining that staff moderates posts and has partnered with animal protection groups.
"We want to make sure that we're filtering out any bad users and not driving them underground."
The site, however, has had trouble blocking some sellers in recent years.
A Nova Scotia woman, Gail Benoit, was banned by the site after Kijiji said it received hundreds of complaints about her. But she continued to try to find ways around the ban by using fake names, different computers, and getting others to post ads on her behalf.
Some of Kijiji's competitors have taken steps to cut out pet sales altogether.
In August, the British Columbia-based site BuySellTrade.ca removed private pet ads altogether. It now only posts pets for adoption via the local SPCA.
Craigslist also no longer allows the sale of household pets. The site allows individuals to find new homes for their pets, sometimes at a small adoption fee.
Puppy mills continue to be a major problem in Canada, despite stronger legislation in several provinces.
Lapointe's home province of Quebec has a particularly poor reputation.
This year Quebec again placed at the bottom of annual rankings conducted by the US-based Animal Legal Defense Fund.
Last week, 90 dogs were seized from a large-scale breeding operation on Montreal's South Shore. SPCA officials said the animals had been kept in unacceptable conditions.
Canada's Humane Society International says it has been working to get provincial governments to introduce and enforce stronger laws against puppy mills.
"They exist for the sole purpose of breeding, again and again, to produce puppies to be sold in pet stores or online, profiting the breeder," says a statement on the organization's website.