@maggiemca
What operating system are you using? What app version?
Sending a message still works for me with the eBay app version 6.259.0.1 on a Motorola phone with Android 15, but I have seen several posts from users having problems sending messages in the last couple of days, so perhaps there is an update that has not reached me.
It might be worth uninstalling the app, restarting your phone and then installing the latest version to rule out a problem with a past update.
If that does not help, try sending a message using a browser instead of the app.