Is it permissible to sell computer software while fearing its use in sin, and does the seller bear the sin for that, or is he not accountable for what the buyer does?

It is permissible to sell programs and devices that can be used for both good and evil, unless it is known or highly probable that the buyer will use them for unlawful purposes; in which case, selling them to that person is not allowed. However, if you are uncertain, there is no sin upon you for selling them, and the sin of unlawful use falls solely upon the buyer. Selling something primarily intended for unlawful use, such as juice to be made into wine, is not permissible if that intention is known or presumed.