Occasionally, PremiumBreed will block a transfer if it doesn't meet automatic validation rules. There are a range of different error messages that you might receive, but they will generally give you a good idea of how to fix a problem.
In this case, PremiumBreed only allows an animal to be transferred from a seller once. This is a built in protection, so the same animal cannot accidentally be recorded as being sold by the same vendor twice. PremiumBreed will identify where the animal has been transferred previously, and give you a link to this entry. From here, you have a few options on how to proceed.