I don't quite understand how a unilateral system would work.  In the second example, buyer puts up double, so 400 gets locked in contract?  What if seller sends an empty box?  Does the buyer reject the transaction and get all 400 back?

What if the seller did actually send his item but the buyer rejects the transaction anyway?  Does the buyer get his money back while the seller loses his item?

I'm sure I must be missing something