Digital archives protect important data collections from failures by
making multiple copies at other archives, so that there are always several
good copies of a collection. In a cooperative replication network, sites
``trade'' space, so that each site contributes storage resources to the
system and uses storage resources at other sites. Here, we examine bid
trading: a mechanism where sites conduct auctions to determine who to
trade with. A local site wishing to make a copy of a collection announces
how much remote space is needed, and accepts bids for how much of its own
space the local site must ``pay'' to acquire that remote space. We examine
the best policies for determining when to call auctions and how much to
bid, as well as the effects of ``maverick'' sites that attempt to subvert
the bidding system. Simulations of auction and trading sessio ns indicate
that bid trading can allow sites to achieve higher reliability than the
alternative: a system where sites trade equal amounts of space without
bidding.