Well several things to take into account.
1.) Lack of communication
2.) Him posting that on facebook without even talking to you prior, especially since he still hasn't contacted you. It's more like he's avoiding you to be honest.
3.) Him listing that animal for sale even though he hasn't paid it off, much less received it.
It would be different if you didn't hear from him and he wasn't on facebook making posts. It could be he may not have access to a phone/computer or whatever, but being he has access and still hasn't gotten with you. I honestly would notify the people that hosted the auction, and I would see about keeping the money as well for boarding and potential sales you may of missed. If the auction people say that's fine then I would just not pursue it further. But the fact of the matter is this whole thing has taken up your time, and caused you potential missed sales.