[coev]

Pawn shops exist and will give you about that percentage for your stuff.

[WillAdams]

For that small subset of "stuff" which is likely to sell quickly at a suitable profit in the local market as understood by the pawnbroker.

[Freak_NL]

That is exactly the economic reason this idea doesn't work. Pawnbrokers know the value of things and how much effort they can invest in any one item. Selling anything they won't take yourself usually means that you'll have to invest a fair bit of your time to sell it. Anyone doing this commercially can't afford that labour unless the item is worth at least something like €100!