Nice project.
Consider using a CloudDB based chat app as the main part of the app.
https://appinventor.mit.edu/explore/ai2/clouddb-chat
Make requests by chatting 'I have a nearly new soccer ball for trade'
Request an interest 'Will trade hardly used baseball glove for soccer equipment.
This acts as a simple bulletin board
Why? Simple; anyone who has app can participate (limit users to students), all students get notifications they missed as soon as the turn their app on (captures posts made when their ipad is closed).
Alternatively, use GoogleSheets or CloudDb as a data base to let users to a add to a list of wanted items or/and items for trade. Much more complicated but may be what the student wants to do. Set up with item/contact info as a List of Pairs.
Difficult. Use either a Firebase or CloudDB with a sign up/log in procedure or a CloudDB restricted app.