Selling a rare username is mostly about presenting the same information a buyer would want to check anyway: exactly what the handle is, how clean the account is, and what the handoff process looks like.
Look at what similar handles (same length, same platform, similar letter/number mix) are currently listed at rather than guessing — see the pricing guides for each platform for a starting reference point.
Be upfront about the account's history and answer buyer questions directly — the same red flags buyers are taught to watch for (evasiveness, refusing basic questions) work against you as a seller too if you're vague.
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.