Paying Method:
1.Buy Now(Ether)
2.Make Offer(ERC20)
3.Make Offer(Ether)
#Procedure:
(1)Buy Now:Seller sign a signature,Buy get the signature and call exchange to finish transaction
(2)Buy Now(Ether):Buyer Deposit Ether into contract,and then sign a signature,Seller get the signature and finish the transaction
(3)Buy Now(ERC20):Buyer sign an order and seller use the signature to finish the transaction.