I’m actually making a course on a marketplace app but unfortunately it won’t be ready for a while.
It is very doable and quite straightforward on Adalo.
Without a course to follow you need to have a play and start going through the various resources and tutorials available for Adalo. It is too broad for someone to really be able to give you an outline here, and you will have to get to grips with the basics. Dive in and i think it will be easy.
But you will need a collection for users (of course), and a collection for items. Relationship items belong to users.
For chat there are chat tutorials on both YouTube and from Adalo that will help you set that up.
There are also templates Adalo.com/app-templates which are similar like the ecommerce template or real estate app