Many animals may end up needing pet medication because of big welfare problems that arise from pets being sold online.
Puppies and kittens are sold for just £1 and a spokesperson for the Blue Cross told the Daily Express that research conducted by the charity shows "major" welfare issues with buying pets online.
Steve Goody from the Blue Cross is quoted as saying: "Irresponsible breeders are using the unregulated forum of the internet to make a quick buck at the expense of animal welfare. There seems to be more protection for people buying a car online than a living creature."
The charity also noted that one woman who bought a four-year-old male Staffordshire Bull Terrier from Gumtree quickly found that she had got an aggressive animal that bit her young son. The decision was made after three weeks to put the animal down.
Gumtree told the Daily Express that it is committed to making sure pet sales are safe and legal, even though it lists animals for sale that cost less than a tin of dog food.