Is the seller sinful for not informing the buyer of an accident that occurred to the motorcycle he sold two and a half years ago, knowing that he repaired it and used it throughout that period without complaint, and what should he do now, especially since both of them are in different countries?

If a significant defect remains in the bicycle after its repair, such that the buyer would refrain from purchasing it if they knew of it, then you must disclose it to the buyer. This is based on the hadith: "The two parties to a transaction have the option [to revoke it] as long as they have not separated. If they are truthful and disclose, their transaction will be blessed. But if they lie and conceal, the blessing of their transaction will be erased."

So, find the buyer and inform them of the defect. If they forgive you and are content with the price, then [all is well]. Otherwise, assess the value of the bicycle with the defect and refund them the remainder. This applies if the buyer was unaware of the defect at the time of sale. If they knew and were content, then nothing is incumbent upon you.

Shaykh al-Islam said: "Whoever sells something adulterated must give its owner the price of the adulteration, or give it in charity on their behalf if returning it is impossible. For example, if one sells a defective item for ten [currency units], and its value when sound is ten, but with the defect it is eight, then one must pay them the two [currency units] if they choose, or return the sold item. If one does not know them, one should give the two [currency units] in charity on their behalf."