What Is Default Ruling In Matters Where There'S No Evidence Of Haram/Halal
This is a wrong placement of the principle of determining a thing to be halal or haram as mutashabeh. Mutashabeh in itself is confusing. The case You cite is about the use of a thing which can be used both as wrong or right purpose. It is the use or action, not that thing which is halal or haram. So one has not responsibility of others’ action.
@Irfan76 and sir if I know that he will or highly likely he will do robbery with this gun, then will I be sinful to sell him gun?