How can edicts prohibiting the sale of devices that can be used for both good and evil be applied, given the inability to track the purchaser's use of the device?

There is no need for investigations. What you know about the buyer's situation, which falls into one of three categories, is sufficient:

1. If you know or strongly believe that the buyer will use it for something forbidden: It is forbidden to sell it to him or repair it for him. 2. If you know or strongly believe that the buyer will use it only for something permissible: There is no harm in selling it to him or repairing it. 3. If both possibilities are equal to you and you do not know what he will use it for: Act according to the predominant use among users. If the predominant use is only permissible, then you may sell it to him; otherwise, you may not.