I'm glad this has come up. I've often been amazed at adverts where the seller seems to assume that his item is SO desirable that a buyer will move heaven and earth to acquire it. Of course we all want the buyer to do everything. Pay first, drive to pick it up etc etc and that is what happens most of the time. In the case of heavy items and those of exceptional value travel to pick up is clearly the preferred option. However if you do post it is up to you to pack well and post by a method agreed by the buyer.
If it is any comfort I have posted and received hundreds of items over the years and-touch wood-I have never had any problems with items I have sent.