As a buyer I often notice that some sellers have unrealistic terms and conditions on their listings and these often contravene Ebay and PayPal policy.
'I am not responsible for items that are lost in the post' Yes you are. You are responsible for getting the item you have sold to the buyer. If it does not arrive all the buyer needs to do is open a dispute with PayPal or Ebay to get their money back.
'Once the item is in the hands of the Post Office it is no longer my responsibility, I will send you the proof of posting and you can claim from Royal Mail' As above, however the seller may find that they send the postage receipt and the buyer opens a claim and gets the refund via PayPal, they are then left with no way of making the claim from Royal Mail which they should have done in the first place.
'I do not accept returns' If the seller is a business seller they are obliged under the Distance Selling Regulations to accept a return. More information is available on the Buying and Selling forums on this subject. However, a private seller who is selling their own personal and unwanted items is NOT obliged to accept a return.