People breeding cats for sale should be licensed and there should be a ban on selling very young kittens to protect their health, a cat charity says.
Cats Protection wants the law changed so that cat breeding is regulated in the same way as dog breeding.
It highlighted a case from Bridgend where two five-week-old kittens falsely advertised as eight weeks old needed care after becoming very ill once sold.
The Welsh Government is consulting on some changes to pet animal sales.
Cats Protection said its centre in Bridgend had taken in the two kittens, named Daff and Dill, after their new owners realised they were unwell when they got them home. The pair of cats had severe diarrhoea and were still trying to suckle.
The owners took them to a vet who said they were five weeks old rather than the advertised eight - the age at which kittens can be safely removed from their mothers.
They also had a huge amount of worms, and jaundice in one case.
The vet said they could die so the owners sought help from Cats Protection, which took them in and gave them vet-supervised care for four weeks.
"We were really shocked at how ill they were," said centre manager Sue Dobbs. "The little girl Daff was particularly bad as she was very cold and very lethargic.
"As well as diarrhoea, the kittens went on to develop flu and we soon discovered they were deaf too.
"It took four weeks and a lot of energy and money to get the kittens back to full health."
The owners were not able to offer a permanent home to the deaf kittens due to their specific needs so they were rehomed, the centre said.
"Our branches across Wales still regularly tell us harrowing stories of kittens bred and sold that are very sick," said Madison Rogers, advocacy and government relations officer for Cats Protection.
"It's vital that the breeding of cats is regulated and the law on selling cats in Wales is updated to avoid such tragedies."
The charity wants the government to regulate cat breeding and to update the law to reflect the fact that most cats and kittens are now advertised for sale online.
It wants this to include a ban on the commercial sale of kittens under the age of eight weeks.
The Welsh Government said in a statement it was currently looking at banning the third-party sale of puppies and kittens - meaning someone other than the person who had bred them - and it would look at the supply chain of the animals to see where intervention would have the greatest impact.
At present there are fewer than 20 licensed pet shops selling puppies and kittens in Wales.
A spokesman added: "The practice of selling kittens and cats online is not a specific part of the consultation, but any relevant comments via responses will be considered."