If you look at how many transactions happen in the used car market across various modes, it's staggering. And there obviously are a LOT of unsolved problems - including valuations, inspections, certification, the transaction itself.
Yes, if the attempt is to do exactly what a used car mart does, only (supposedly) better, there's no play. But there's immense scope for providing assistance, tools to enable and make smoother bits of this whole food chain.
To say there's no market for this might be a limited view, and likely wrong.