eBay stops supporting older versions of the app and after a certain amount of time you may find that your phone is not compatible with newer versions of the app.
If an old version of the app was already on your phone you could use it, but you'd encounter a lot of errors. Since your phone doesn't currently have the app and you can't download it, that means the available versions are not compatible with your phone.
Your phone was released 10 years ago. In the tech world, it's a long extinct dinosaur. (Edit to add: I just looked it up and Samsung stopped supporting your phone 4 years ago so even your OS is likely out of date.)
Do you have a more recent tablet you can use or a computer? If not then my suggestion is to use the eBay website through a browser on your phone since you can't access the app. At least until you upgrade your phone.
