Digital archives can best survive failures if they have made several
copies of their collections at remote sites. In this paper, we discuss
how autonomous sites can cooperate to provide preservation by trading
data.  We examine the decisions that an archive must make when forming
trading networks, such as the amount of storage space to provide and
the best number of partner sites. We also deal with the fact that some
sites may be more reliable than others. Experimental results from a
data trading simulator illustrate which policies are most
reliable. Our techniques focus on preserving the ``bits'' of digital
collections; other services that focus on other archiving concerns
(such as preserving meaningful metadata) can be built on top of the
system we describe here.