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?