there are many ways,
1) simple search and check good profiles.
2) sometimes buyers use filter options to find good sellers.
3) they directly come from google search results. (Fiverr does SEO and in a search result, Fiverr have a separate page for quality sellers of that particular category).
4) paid ads, Fiverr promote separate page for quality sellers of that particular category.
5) if the client doesn't like any seller, Fiverr send them an email with a list of quality seller.
6) if the client doesn't like any seller, Fiverr send a notification with a list of quality seller.
please note: all 6 ways I mentioned here are based on my own experience. it's not officially claimed.