How much did you pay for your new mattress?
Assuming it's a traditional construction 60x80 queen, not tempurpedic or something else, I assume you paid well <$500 for it. Therefore I think a used mattress, and by used I mean previously sold regardless how many nights slept on it, I would think it's worth a 1/4 of new.
Personally I would never buy a used mattress, would you? Hence the extreme depreciation.
Just my opinion, others may see some value it and all it takes is one of them to find your add.
2007 Chevy Duramax LMM/Allison
2012 Jay Flight 29QBH
2012 Jay Flight 26BH (Sold)