I don't understand, Firefox has this functionality. For example I have it set up so if I type 'y cats' it will search Youtube directly for cats. You can do the same with ebay. Similarly if I type 'yout<tab>cats' it will do the same search. Also, if you like it so much, you can configure Firefox to not put things like bookmarks and history before search results in tab-complete search suggestions. Firefox's address bar is strictly superior as far as I can see. The only thing it's missing vs Chrome is dark patterns.

1. In Firefox settings, tick "When using the address bar, suggest: Search engines".

2. Find an eBay search engine you like, install it in Firefox.

Now, when you type eba{Tab} Firefox will suggest the search engine you installed, and your continued typing will be as a search in that engine.

I don't use this for eBay, but I do use it for Rust, typing e.g. rust{tab}iterator takes me to the documentation for std::iterator and related entries.