You can't sell items back for credits and you can't trade them with other players. (escapistmagazine.com)
You can often find used ones in good condition for sale online, or for trade with other parents for different sizes. (herbabystuff.com)
It's good that we can set that sort of thing aside and engage in trade with other countries, for example, that choose to run their countries differently from ours. (canadianbusiness.com)