Is it permissible to sell a poor-quality computer at an exorbitant price to the seller who deceived me with it, knowing that he will use it to deceive someone else?

It is permissible for the questioner to resell the computer to the one who sold it to him, and the ruling does not change if he knows that the seller will defraud another person. The questioner is only responsible for his own actions, and the sin of the seller is upon himself, as Allah Almighty says: "And whoever earns a sin, he earns it only against himself" and "You are not tasked except [with regard to] yourself." The questioner should advise the seller to be truthful and trustworthy, and enjoin him to do good and forbid him from evil. If he does that, the seller's sin will not harm him thereafter, as Allah Almighty says: "O you who have believed, عليكم أنفسكم; no harm will come to you from those who have erred if you have been guided."