Complaints about illegal puppy sales have almost trebled in a year, according to the Scottish SPCA.
Rogue dealers sell pups for up to £2,000 each and can make as much money as drug dealers, with far less risk, say experts.
The Scottish SPCA received 510 reports from the public raising concerns over puppies being sold illegally last year, compared with 186 the year before.
The online trade has been valued at around £13 million a year. It is booming because of soaring demand for trendy breeds such as French bulldogs and pugs, with pups being sold for up to £2,000 each. But, because of how they are bred and sold, one in three become ill and one in four die before they are five.
Experts say puppy farmers can make similar profits to drug dealers while risking a fraction of the punishment if caught.
And they have warned that rogue dealers are deceiving buyers by adopting tactics such as renting Airbnb properties to make it appear as if dogs come from a good home.
Mike Flynn, SSPCA chief superintendent, said much of the trade in Scotland is now driven by supply of cheap dogs from the Republic of Ireland.
He said: “We have a huge amount of people dealing in puppies that have come from puppy farms elsewhere.
“One of the problems is when you look at the amount of dogs required to keep up the dog population in Scotland, there are not enough licensed and Kennel Club-assured breeders to supply that demand. Then you get a craze for French bulldogs or pugs and the home-grown supply is not enough.
“Over the years, we have been urging people not to buy over the internet, but the demand is such that people will do it.
“There are only a small amount of French bulldog breeders in Scotland, for example, and if you phone them up there will be wait of 24 months. They have already sold the next three litters because they are reputable and you know you are getting a high standard.
“When people get that information, they will go on the internet and think I can get a French bulldog next week. But it’s people buying misery.”
An analysis carried out for the Scottish Government found puppies being sold online for an average of £800 – adding up to an estimated £13m in a year.
Mr Flynn said a litter of six puppies could be bought in southern Ireland for around £300, then sold for £750 each.
Welfare issues that can result from puppy farming include health problems for the mother from being bred too often and dogs being kept in poor conditions. Pups can also be prone to poor physical health and have chronic behavioural problems from a lack of socialisation.