Is it legal for pet shops/stalls to sell kittens in cages?
If you are in the UK, it is local councils who issue licenses for traders to be able to sell animals. Each council has their own standards and regulations, so it might be worth contacting your local office to see if this market trader has a license, and if the conditions in which the animals are being kept are approved. It might also be worth asking your vet to confirm in writing his findings about the kitten, so that you could make an official complaint against the market trader. Hopefully, they may be prevented form selling any more animals in the future.