Most online auctions attract more bidders if the buyer knows the shipping amount up-front. While some sellers are tempted to make a profit on their shipping fees, experienced buyers are often sellers as well, and can be turned off by fees that seem markedly higher than what they know shipping companies will charge.
Online auction sellers depend on their feedback ratings to help generate more business, so it is to your advantage to become known as someone who ships quickly and packs goods well.
To protect yourself as a seller, use a traceable shipping method with shipping insurance, so an unscrupulous buyer can't claim the goods never arrived. Keep records of every sale, and of the messages that pass back and forth between you and the buyer.