"Clean" describes a username made up of plain letters with nothing else tacked on — no digits, no underscores, no leftover numbers from a taken name. It reads the way you'd type it if you had a totally free choice.
Most available usernames today aren't clean — they're the letters you wanted plus a string of digits because the plain version was already taken. A clean handle skips that entirely, which is part of why it can't just be registered and has to be bought.
The two often overlap but aren't the same thing — a clean username can be long (a full real word), and a short username can still include digits. Length and cleanliness are separate factors in what makes a username valuable.
Yes — it's sometimes used loosely for an account with no flags/history issues too. Context usually makes clear which one is meant.
Open the vault, pick a listed username, and DM the contact handle shown on the site to arrange payment — there's no cart/checkout, it's a direct conversation with the seller.
Crypto-only. See the Terms of Service for the full payment and delivery policy, including the sale-finality and 72-hour issue window.