Im new, how do i get the cost from the buyer.

If the postage cost was not noted in the auction, you either ask a question if the auction is still running  or email the seller if the auction has finished and you've won it.

It is best to have your postage stated on the auction when you list it. Much smoother for both you and the buyer.  

Kia ora knightman, if your listing has already closed you can email the buyer to arrange shipping and either send them you bank account details, or set up a new listing for them that includes the postage costs (use their member name in the title and let them know). For smooth future trades the advice above from AJ and Callum is spot on.