For me personally, it really comes down to what is being done to the card and why it is being done. I don't think there's many that would complain about someone simply wiping dust/dirt off the surface, as they would likely do the same, before putting the card into their own collection.
When someone is needing to break out wax, polish, liquid, brushes, reshaping tools,.... to "clean" a card, that's where I think it moves into the alteration and disclosure territory. You can do whatever you want, but if you're doing so to improve the value (restore the the card), it's only fair to inform the buyer.
I've shared this before and it's on topic, so thought it might add to the discussion. While it doesn't specifically mention "cleaning", there's actually a law in California that requires disclosure of altered/refurbished trading cards. Up to $5000 penalty per card, if sold without disclosure of the work done. Would "cleaning" fall within this description?
“Altered or refurbished” means repair work which has been performed to enhance the value of the sports trading card as a collectible. This work includes, but is not limited to, filling in holes, building new corners, ironing out creases, or touching up the pictures or borders on the sports trading card.
https://leginfo.legislature.ca.gov/face ... nNum=21671
ARTICLE 7. Sports Trading Cards [21670 - 21672] ( Article 7 added by Stats. 1992, Ch. 1120, Sec. 2. )
21671.
(a) Any sports trading card that is altered or refurbished shall be accompanied by a certificate stating the exact work done to the sports trading card, the date the work was performed, the cost of that work, and the name, phone number, and address of the person who performed the work.
(b) Any person or agent thereof, who knowingly sells or trades a sports card in violation of subdivision (a), shall both:
(1) Refund to the buyer, the full amount paid for the altered or refurbished sports trading card or the full retail value of any nonmonetary consideration received in exchange for the altered or refurbished sports trading card, or both.
(2) Be liable to the buyer for a civil penalty not to exceed five thousand dollars ($5,000) for each violation. Each card sold represents a separate and distinct violation.
(Amended by Stats. 1995, Ch. 360, Sec. 1. Effective January 1, 1996.)