I do most of my hunting between the trade search thread and FTT, so sometimes I have an idea why they cancel and other times I don't
There are trades where person A was being offered a sunjewel by person B and person A was told it was overpay but they still canceled. While this can still be posted for proof, I don't know why or how the skelebun was valued to reach a conclusion... That's why I didn't post many, but you're all welcome to your research. I still stand by putting it between a dogtag and moonswirl since there wasn't a lot of successful trading evidence (and it's not the rarest pet on the site either, it shouldn't be at the tippy top right now).
There was an auction where none of the bids (3 total) were accepted, but all were worth around 4-5 nons. I wish they'd posted the winning trade so I had evidence for why they were all canceled... But I don't ;-;