[mg]

I wish there was something like a second hand store where you can just drop your stuff for 10% of its price on eBay.

I don't want to deal with anything. Not wait for someone to pick it up. Not wrap it in a parcel and bring it to the post office. Not sign up for some service. Nothing. Just drop my stuff, get a little bit of money and am done with it.

[bombcar]

They are (or were) called consignment stores and most of them would be knowledgeable in their area to give you quick cash.

If it’s valuable, you could pawn it.

Most people just drop it on GoodWill.

[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!