For case citations, texts and other lex mercatoria material on silence by offeree:
http://tldb.uni-koeln.de/TLDB.html
Click TLDB's "List of Lex Mercatoria Principles".
"Silence by the offeree does not in and of itself amount to acceptance unless the offeree begins with the performance of his contractual obligations or is required to reject the offer due to a long-standing business relationship with the offeror or is subject to a practice which the parties have established between themselves or a trade usage requiring rejection of the offer ('qui tacet consentire videtur')."
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Pace Law School
Institute of International Commercial Law - Last updated January 28, 2003