Characters may purchase any material available in their surroundings by spending an amount of zenit equal to its value. When it comes to Projects, step 3 on page 134 of the Core Rulebook abstracts this process, but the group might prefer to purchase specific materials and keep them for future use.
The Game Master has the final say on which materials can be bought in any given area, at which cost and in which quantity. However, they should involve Players who control characters from that region or have studied it, or who play Classes like Tinkerer, Merchant (see page 158), Loremaster, or Wayfarer.
If there is an artisan, merchant or other potential buyer available, Player Characters can usually sell materials at half their normal value in the same way as for normal items (see Core Rulebook, page 124).
Continuing from the previous example: if the group ever decides to sell that titan carapace they retrieved, they would earn 500 zenit.
On the other hand, using a material to cover the cost of a Project (see Core Rulebook, page 137) or to forge items as described on the next page will let you use its full value, showcasing its full potential.
These options involve PCs in the process of transformation, death, and rebirth typical of natural fantasy, but must not be used to reenact colonialist fantasies or gratuitous cruelties towards animals and living beings.