maybe there is a way and i'm just missing this, seems to me that if i'm thinking "oh i've found this wonderful item but i don't need it, perhaps i can put it up for sale to get that other item i do need" then i'mi at a loss because you currently need the item you want to have used as payment.
for example i can't place an item for sale for 10 diamonds if i only have 9 diamonds. in order to gain something from a sale the player selling already needs whatever it is they wish to have.. be it 10 diamonds or that illusive nether star