There are probably ways you could make it manageable and less morally repugnant.
1) Let people set their own price. Then it's not the platform valuing you, it's how much you value your own time.
2) Provide "auto-pricing" as an optional feature. Use an imaginary unit - call it "stamps" and provide ways to earn "stamps" on the platform besides just buying them.
"How many stamps should it cost for somebody to send you a letter? Or you can use one of our auto-managers - do you want to receive lots of letters or only a few?"
You could still have some kind of invisible credit score under the hood even for users that don't use the auto-pricer - people with good credit get plentiful free stamps.
Then you'd have to get into a whole Google-style graph-based-ranking but for people, which I assume sites like Twitter are already doing under the hood.
Of course, the problem here is that you'd have a plethora of alts to farm free stamps, which would clutter the search list...
Everything is hard.