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

unilateral is uneven deposit (usually its double deposit) do to the review rating of the seller or buyer the deposit can be lower if both parties agree because of user rating.
Try the client try it on a small transaction you would understand it more, all your question could be answer by using it.