I think you MUST disclose that this card came back from PSA as Altered. Look at this thread here:
Link. In this thread, the seller cracked a card out of a SGC 30/2 holder, and then advertised the raw card as EX/EX+. Now are people saying that doing this is perfectly legitimate? After all, it is only a TPG's OPINION that the card is a 2 (Good). Grading is subjective, so if the seller wants to crack it out and advertise the card as Excellent, this is perfectly okay? I say this is dishonest, exactly like the case in this thread.
The case where someone cracked out a card 15 times before finally getting the grade he wanted is different. In that case, a disinterested third party gave their professional opinion on that grade. Sure, they gave a different opinion the 15 times before. However, they don't have a dog in this fight. In this case, the seller has an obvious conflict of interest because he will directly profit from the own grading that he is giving, especially when he knows a TPG gave a different assessment. He should say something along the lines that "PSA gave this card an Authentic/Altered grade. However, I believe the card is not trimmed. Look at the bottom edge closely, and you as the buyer be the judge."
I would add that if you KNOW the TPG is wrong, then you should disclose that. For example, if there is a mark on the card that the TPG misses, you should disclose it.