If a person whose clothes were bought with stolen money, like a thief, even though he repented, supplicates with those clothes on, will his supplications be answered by Allah?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Repentance wipes out all previous sins, and the repentant person is the same as the one who is sinless. Hence, if the mentioned person has sincerely repented to Allah and then performed the prayer wearing those clothes, his supplications shall be answered as long as the conditions required for having one's supplications answered are met and the impediments are absent.

Al-Haafith Ibn Hajar wrote:

"Allah answers all supplications, but the nature of the answer differs. Sometimes the supplicant is granted what he asks for, and sometimes he is given something else to substitute it. ‘Ubaadah ibn As-Saamit narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said, 'Whenever a Muslim supplicates Allah, He accepts his supplication or averts an equivalent evil from him.' [Al-Haakim and At-Tirmith: authentic]

Abu Hurayrah narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said, 'There is not a man who calls upon Allah with a supplication except that he is answered. Either it shall be granted to him in the worldly life, or it shall be reserved for him in the Hereafter.' [Ahmad]

Abu Saʻeed narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said, 'There is no Muslim who makes a supplication, as long as it does not pertain to anything sinful or the cutting of ties with kinship, except that, due to it, Allah will give him one of three things: either He will immediately respond to his supplication, or He will save it for him in the Hereafter, or He will prevent an equivalent evil from befalling him.' [Al-Haakim: authentic]

This is a second condition required for the supplications to be accepted. There are other conditions as well, such as consuming and wearing only what is lawful, as evidenced by the hadith which reads, '...how then can his supplication be accepted?' (mentioned in full below) Another condition is avoiding impatience, as underlined in the hadith which read, 'The supplication of every one of you is granted if he does not grow impatient and says: I supplicated, but it was not answered.'" [Fat-h Al-Baari]

It should be noted that wearing unlawful clothes is among the impediments of having one's supplications answered. This is evidenced by the hadith which reads, "He then mentioned a man who travels for a long time, with his hair disheveled and covered with dust. He lifts his hand towards the sky (to supplicate), saying, 'O Lord, O Lord,' whereas his food is unlawful, his drink is unlawful, his clothes are unlawful, and he was nourished with what is unlawful. How then can his supplication be answered?" [Muslim and others]

Al-Mubaarakfoori said, "It is deduced from the hadith that what a Muslim eats, drinks, and wears should be purely lawful, without the slightest doubt, and that whoever wishes to supplicate Allah should fulfill these conditions with greater reason." [Tuhfat Al-Ahwathi]

Allah knows best.