Taking no responsibility

I don't understand how a seller is able to not accept responsibility for the items they are selling. Otherwise, what is to stop people just sending out second-hand damaged goods and claiming that they were fine when they left them?

Actually I'm just going to add to this! Have just read that if you donate a bag of items to a charity shop, etc and something in the bag doesn't work and the customer demands a refund it could be up to you to pay it. That's crazy. I know it's highly unlikely you would be traceable but how on earth does the shop not get the responsibility for this? Answers on a postcard please ...

I promise I'm not making it up! You would think it was obvious wouldn't you but apparently not.

Gift aid ? beware the hidden catch | Money | The Guardian