He bought some clothes from a store, then found out that it sells stolen goods

Firstly,

If you find out that this store sells stolen goods, it is not permissible to buy from it, because the stolen goods are not the property of the thief, so it is not valid for him to dispose of them.

The Permanent Committee for Ifta’ was asked: Sometimes stolen goods come to the market, and what indicates that is the fact that the seller is in a panic, and he does not know what something contains, or the type of device and how to operate it; or the goods are sold for an extremely low price; or the seller does not know where he bought it from. What is the ruling on buying such goods?

They replied: If a person is certain that the item offered for sale is stolen or has been seized by force, or that the one who is selling it does not have ownership of it in a shar‘i sense, and he is not acting as a proxy for its owner, then it is haram to buy it, because buying it comes under the heading of helping in sin and transgression, and causes loss of the item to its real owner, and because of what that entails of wronging people, approving of evil deeds and sharing in the sin of the one who stole it. Allah, may He be exalted, says (interpretation of the meaning): {And cooperate in righteousness and piety, but do not cooperate in sin and transgression} [al-Ma’idah 5:2]. Based on that, the one who is aware that this item is stolen or was seized by force must advise the one who stole it, gently and with wisdom, to give back what he stole; if he does not give it back and he persists in his wrong deeds, then he (the one who is aware of the theft) must inform the authorities of that, so that the perpetrator may be brought to justice and the goods may be restored to their rightful owner. This comes under the heading of cooperating in righteousness and piety and, moreover, will deter the wrongdoer from his wrong doing and will support both him and the one who was wronged."(Fatawa al-Lajnah ad-Da’imah 13/81).

Secondly:

The one who bought goods knowing that they are stolen must return them and get his money back, because the transaction was not valid.

As for the one who bought them, then it occurred to him that they might be stolen goods, but he was not certain of that, he is not obliged to return them, because in principle the transaction was assumed to be valid.

And Allah knows best.