...a retailer were to have a product that by itself is just a plain everyday product. A second company has another product in the same way. A third party that is not affiliated with either of the companies comes up with an idea to use both products together to make a useful tool for something, but that something, though useful, is not what either of the products were intended for. However the third party does not buy, assemble or market the tool, just lets it known online and otherwise that both products can be bought and a tool assembled by the purchasers of the products.

 If people buy the two products and use the tool, but one or two do so in a manner way beyond either product's specification, and someone or some property gets damaged, who would the lawyers go after? Or go after first anyway? Any chance the purchasers would be on their own?