Idk. If an undercover FBI agent can convince you to order C4 with intent to blow up infrastructure, who says someone else couldnt. Id rather it be an agent locking him up, than real seller, while an agent dont notice.
It seems to me your argument can be extended to justify any form of entrapment: "if an undercover police officer can convince you to buy a gun with the intent to rob someone at gunpoint, who says someone else couldn’t?"
If an undercover cop sells you a gun knowing you want to use it to murder your spouse, you should definetly be charged with both, illegal purchase of firearm and intent to murder, regardless whether you actually to managed to murder someone first.
Does it matter that all someone (be it undercover cop or criminal) proved was you are able to be convinced to murder your spouse AND buy a gun? In my books you either charge the person at that point (for intent to murder), or it will be too late.
Same applies to "suspectible to destroy". They proven their intent AND purchased C4... Shall we give them benefit of the doubt until it actually goes boom?
I am sure many things can be prosecuted before they even happen (like intent to murder as oposed to murder itself), and those should apply regardless of who talks you into intending to carry out the crime