Crossroads demons are Hells salesmen, they’re the ones who broker the deals for people’s souls. So, say you really wanted that book shop you were talking about, like so bad you’d be willing to sell your soul for it, you’d summon a crossroads demon and they’d give you a contact. You get your bookshop and ten years to live. Then when your times up they’d send hellhounds to collect your ‘payment’. Then of course your soul is damned to spend eternity in hell. They’re not usually so up front about that part.