If you are using the Android version of the eBay app, use the three bar menu icon on the left side, scroll down to the Settings menu, and from there you should see a sign out option.
If you are using the iOS version of the eBay app, there should be a My eBay link at the bottom of the screen that should allow you to reach a sign out option.
There may be limits to the number of times you can sign out and sign back in using the eBay app each day, so if you need to quickly switch accounts, you may want to open a browser and sign in to one of your accounts that way rather than signing out and back in using the eBay app.
