I think about this a lot when I trade in VMK...it is so spur-of-the-moment for me, I get nervous that it is a "bad" trade because I don't know the value of what I have...but then I think to myself...I don't want this item, and I want what they have, so it must be a good trade. I try not to think about what they are going to turn it around for and if I should have saved it and who is laughing at me for trading what...because tomorrow is another day and I will hopefully have another thing and keep trading. Everyone has their "thing." Mine is (currently) pink furniture, and not Herbie Posters...so someone else who wants all the posters and doesn't care about a pink Nemo chair is a good match for me, regardless of relative "worth."
At least that's what I keep telling myself.