Well it is asking for trouble, as you have no idea if these animals are healthy, and even more important if they have been stolen (which does happen).
If you want quality stock for either dairying/household milk and/or meat the British Goat Society (mostly dairy breeds), or the Boer Goat Society (meat goats only) are your starting point.
If you are wanting pets then with Pygmy goats it is even more important to buy registered (both with DEFRA and the breed association) stock, as these are notorious for "going missing" or being bred without the relevant paperwork.
Always ask for the CPH number in the first instance, then the UK herd number.