Praise be to Allah.
Firstly: selling decorative birds like parrots, colourful birds and nightingales for their voices is permissible, because looking at them and listening to them is something that is permissible. There is no text in the sharee’ah which forbids selling them or collecting them; however there are texts which may be understood as meaning that it is permissible to keep them in cages, so long as they are fed and watered and given whatever care is needed. Among these texts is the hadeeth narrated by al-Bukhaari from Anas, who said: “The Prophet (peace and blessings of Allah be upon him) was the best of people in attitude. I had a brother called Abu ‘Umayr, who had just been weaned, I think. When (the Prophet (peace and blessings of Allah be upon him)) came, he would say, ‘O Abu ‘Umayr, what happened to the nughayr?’ – a nughar [young sparrow or small bird] with which he used to play.” This nughar is a type of bird. Al-Haafiz ibn Hajar said in his commentary (Fath al-Baari), when enumerating the things that we learn from this hadeeth: “… it is permissible for children to play with birds; it is permissible for parents to allow their children to play with things with which it is permissible to play; it is permissible to spend money on permissible things to entertain children; it is permissible to keep a bird in a cage or to clip a birds’ wings – for one or the other must have been the case with Abu ‘Umayr’s bird, and whichever one it was, the other is also implied in the ruling.
There is also the hadeeth narrated by Abu Hurayrah (may Allah be pleased with him), in which the Messenger of Allah (peace and blessings of Allah be upon him) said: “A woman entered Hell because of a cat which she detained but she did not give it food or water, and she did not let it free so that it could eat of the vermin of the earth.” (Narrated by al-Bukhaari in al-Saheeh, 4/100,152; Ahmad, 2/261). If this is permissible in the case of cats, then it is also permissible in the case of birds etc.
Some scholars believed it to be makrooh to keep them in cages. Some believed that it was not allowed, and said: because people do not need to listen to their voices and look at them. This is an unnecessary luxury and soft living, and it is also foolishness because it means that one is enjoying the voice of a creature that is calling out in longing to be allowed to fly and grieving that it cannot fly freely in the open air – as was stated in the book Al-Furoo’ wa Tasheehuh by al-Mirdaawi, 4/9, and al-Insaaf, 4/275.
Secondly: among the conditions of valid sales is that the thing to be sold is something which it is permissible to use and benefit from, even if there is no necessity as such. There is no benefit in snakes, which are indeed harmful, so it is not permissible to buy or sell them. The same applies to lizards – there is no benefit in them, so it is not permissible to sell or buy them.
Thirdly: it is not permissible to sell predators such as wolves, lions, foxes or any other fanged carnivore, because the Prophet (peace and blessings of Allah be upon him) forbade that, and because it is a waste of money, which was also forbidden by the Prophet (peace and blessings of Allah be upon him).