Prices offered in itembuildr co-op are like the other stores, if you have a higher frugality you can buy it cheaper. Not everyone wants to re-spec their character to max frugality, so it makes sense to buy it from someone else who could get it at the lowest possible price.

Sellers’ motivation: Gavel Time e-peen (and to be nice).

EDIT: Ninja’d while double-checking whether frugality works as expected at co-op.