As stated on this same list, I have been in contact with Marshall.  After
reading your post I wrote them asking this question and their response
is, in no way do they ever take ferrets back, for the simple reason that
it might introduce disease into their breeding facilities.  So since they
do not accept them, they do not destroy them.  They do passively assist
in finding shelters if called upon by a store, but for the most part
leave it up to the individual store as they know their area better.  I
have written to PETCO to ask what their policy is and am still awaiting a
reply.  I know "official policy" can mean little some times, but without
hard evidence to the contrary, I would tend to take their word for it.
 
If you go to their website www.marshallpet.com and then to "contact us"
either Rosemary or Pete will call you to answer your question.  I wrote
an email and Rosemary called me about 5 minutes later, so they are happy
to answer questions.
 
James, Melissa & Kassidy
 
Loving:  Bebe, Riley
Missing: Wheezer, Pym, Rikki, Jako
[Posted in FML issue 4182]