The latest version of the eBay app should be available in the Google Play Store.
If you are having problems getting the latest version of the eBay app, make sure you have the latest version of Google Play Store and Google Play Services.
https://support.google.com/googleplay/answer/9037938?hl=en
If you are still unable to get the latest eBay app for your Chromebook, you should still be able to use a browser to go to ebay.com.
One possible complication would be if the app is set to open eBay browser links automatically; there is a setting to disable this.
The last time I used a Chromebook, the procedure was something like this, though it may have changed since then:
Open Chromebook Settings, scroll down to Google Play Store and select it. Select "Manage Android Preferences". From there go to Apps: eBay: Open by default: Open supported links: select "Don't open in this app". Then back out of the settings and browser links should open in the browser rather than the eBay app.
Or else, open Chromebook Settings, and select Apps. Select "Manage your Apps". From there go to eBay: More Settings: Advanced: Open by default: Open supported links: "Don't open in this app".
