When someone is looking at your item, they want to find out everything they need to know with the least amount of difficulty. It is very important to include as much information as you possibly can about the item you are selling. If the buyer wants to bid on your item but wants to know the postage cost before doing so, they dont want to have to email you as this takes time. If they are wanting to bid on your item and it is only a short time till it finishes, you may not be able to answer the question regarding postage before the auction ends and you may therefore risk losing a buyer. Even if it is not possible to provide an exact postage quote due to the amount changing depending on the destination to which it is to be posted, you could provide an approximate and just suggest that the buyer emails you. That way at least they have a reasonably good idea about postage so if you are unable to get back to them before the auction ends they may still bid on the item. Also, the other option is to add a postage calculator to you listing so people can enter their postcode and get a postage estimate.
With postage also it is important to offer options. Some people like to have the item registered so as to ensure the safety of their item and money should anything happen to the item in transit (or go missing) The option of express post also is a great idea as customers may need the item in a hurry if they are purchasing it as a present.