I have found that it can be hard to find what you are looking for second hand on sites like ebay, especially if you model internationally like myself. I am an American who models British OO scale and it is a daunting task trying to find what I am actually looking for. If there was a dedicated buying and selling section on the app with full categories it would make expanding collections much easier. Categories/filters for listing could include the following:

Scale

Brand (Separating region based companies such as Rapido Trains and Rapido Trains UK

DCC Status (DC, DCC Ready, DCC Fitted, & DCC Sound)

Type (Steam, Diesel, Electric, Coaches, Wagons, etc)

Price

Condition (All with descriptions in the app, New, As New, Great, Good, Fair, Non-runner etc.)

Plus more if anybody else has ideas.

I think instituting the buying and selling feature would be incredibly beneficial for everyone whether they want to expand or thin their collections.