I don't know how we could incorporate this into the rules. There are so many facets to buying and selling a live animal such as fish, I don't know that it would be something that we would want to get into unless there is some kind of fraud involved.
I would be more inclined to incorporate a review or rating system to let people judge for themselves. I think that might open another can of worms, but I think over time, site members would be able to tell the difference between a disagreement and a person who has a habit of not acting honestly.
In the meantime, let's keep in mind whether you're buying or selling, try to be considerate of the other party. If something happens, regardless of what it is, at least contact the other party so they can make a decision about whether to continue with the transaction or move on and be able to do so in a timely fashion.