Does the painting work we did in a house with a defect (dampness leading to peeling paint and a faint odor) count as aiding in deception, knowing that we do not know the buyer, nor do we know if the homeowner informed him of the defect, and that the homeowner did not hide the matter of the paint or dampness from visitors?

There is no harm in painting old houses and treating dampness, even if the owner intends to sell them, as long as the painting is not done with the intention of concealing a defect from the buyer. If you know that he intends to conceal a defect, it is obligatory for you to refrain from working to avoid assisting him in deception, in accordance with the Almighty's saying: ﴿And cooperate in righteousness and piety, but do not cooperate in sin and aggression.﴾ However, if you do not know his intention to conceal a defect, there is no sin upon you. The default is to assume the soundness of Muslims' affairs and to think well of them, in accordance with the Almighty's saying: ﴿O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin.﴾ And the seller of the house must disclose any defect in it to the buyer, and painting it without disclosing the defect is considered forbidden deception, in accordance with the Prophet's (peace be upon him) saying: "He who deceives us is not of us."