How do I seach for a specific selller when im at the Gameflip home page?
I don’t think you can. Thats why I include “vanilla” in my listings so people can search for me 
If you have a favorite seller then you should bookmark their profile page in your browser, which would show all their listings.
Or better yet, “Follow” the seller. You can receive notification (push notification if you have mobile app) when there’s a new listing from the seller. You can mute it if you receive too much notification from the seller, then it’s effectively a bookmark for the seller.