Jason,
I would say that one would need to be up front about not having the full story about the snake he is offering for sale (no papers, etc), and I would have listed it as an "axanthic possible" until I was certain about what I had.
However, the fact that the person to whom you sold this animal has waited a month before saying anything to you about it key here. One month is too long to wait to complain just because you're upset about the animal's looks, and it is too short to say that you've tested it's ability to pass on axanthism through breeding trials.
So, in this particular situation, I would say that you owe no compensation whatsoever. That being said, I would urge you to be as absolutely clear in your advertisements as you can in the future. I'm not saying you did anything wrong, but if that animal looks axanthic to "several people" say that in the ad and leave it at that. <img src="http://www.faunaclassifieds.com/iB_html/non-cgi/emoticons/wink.gif" border="0" valign="absmiddle" alt='
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