That really depends on whether the bomb making part is specific to bombs, and if their purchase of that item could be considered legitimately suspicious. Many over the counter products have the potential to be turned into bombs with enough time or effort.

If a murderer uses a hammer, do you think the hardware store they purchased the hammer from should be liable?

You can make crude chemical weapons by mixing bleach with other household items. Should the supermarket be liable for people who use their products in ways they never intended?

Everything needed to make a bomb can be found at your local Walmart. Nobody blames the gas companies when something gets molotoved.

Maybe if the tool’s singular purpose was for killing. I think guns might be a better metaphor there. Explosives have legitimate uses and if you took the proper precautions to vet your customers then it’d be hard to blame you if someone convincingly forged credentials, for example.

I would say the supplier is culpable if the tool supplied is made for the purpose of the harm intended or if the supplier is giving the tool to the person who does the harm with the explicit intent for that person to use it for that harm. For example, giving someone an AK-47 to shoot someone or a handgun/rifle with the intent that the user shoot someone with it. If the supplier gives someone a tool to use for one legit purpose but the user uses it for a harmful purpose instead, I don’t think you can blame the supplier for that. For example, giving someone a knife to cut food with, and then the user goes and stabs someone with it instead. That’s entirely on the user and nobody else.