The somewhat cryptic post seems to suggest that a different person with a different pet actually did the trade, or similar, and the player was scammed/defrauded this way.
Somewhat different topic - - it would definitely be best if pets with buffs of any sort on them had a single. obvious indicator in the trade window. An indicator which communicates, clearly and positively, that the pet is buffed by an outside source, and perhaps which buffs it has, so that it cannot possibly be misunderstood by buyers. Will UO see it implemented however? sigh.
Assuming such a thing isn't happening tomorrow - - it's possible to figure out with some pets - - NOT ALL - - if they have been improperly buffed for the point of making a fraudulent sale. Keep an eye out for stats which are higher than the normally tamed maximums, such as higher than 125 dex/stam, or maybe int that is higher than that particular pet type can ever have. It's easiest to tell if a trained pet has been buffed because the treat is more likely to buff the pet's stats higher than they should be.
It's impossible to tell by looking at the stats in all cases, and that's why there should be something to make it obvious during a trade. Specifically, a low-stat, untrained pet can be made to look like a partly-trained, high-stat pet. Or, a regular pet can be made to superficially resemble an old "curse-tame" (which can be a high-dollar pet these days).
So yeah. imo this whole thing shouldn't be an issue, but it is, and mebbe one day this stuff gets taken seriously and cleaned up.
It's worth mentioning that the stance on player fraud should be different than it is now in UO. There seems to be a customer service standpoint on player-to-player fraud that is lackadaisical and I think that is out of place (and we can ignore the fraudsters and scammers who are loser enough to attempt to defend such activities in this MMO). When a player cannot call guards on a thief who stole from them, the player needs to be able to page the game's GMs and have the situation resolved properly.