I believe money shouldn’t be involved.
Just match sellers and buyers together and let them figure the terms and transaction out themselves.
All that is needed is a way to find what you want and a solid system of building trusted profiles with ratings and such.
Not a simple task, but keep the money out and it will all be easier.
Been using it on my phone and browsers for maybe a year. Works great.
Browser plugin can be a bit shaky at times, but that might just be some side effect from my config of LibreWolf or something else.