Check out this no bullshit article:
http://www.cracked.com/article_22110_5-horrifying-realities-testing-drugs-lab-animals_p2.html?utm_source=facebook.com&utm_medium=referral&utm_campaign=crackedfbfanpage
Direct quote: "When labs aren't breeding their own animals to prod and butcher, they might be going after your old pets. Breeding dogs is expensive and time-consuming. However, there is a ready-made source of free dogs and cats: the local animal shelter. "Class B" or "Random source" animal dealers purchase animals legitimately from shelters or unscrupulous hobby breeders and send them to labs. Less reputable dealers, or "bunchers," will gather animals from "free to good home" ads on services like Craigslist, often posing as legitimate families seeking pets, and then sell them to labs."
If you care, retain the information of the new owner.
Have them sign a document stating they will return the animal if you find later that the home is not reasonable caring for the animal. A new owner should have no problem with.