A display name is the freely-editable name shown at the top of a profile — you can set it to almost anything, and it doesn't have to be unique. A username (the @handle) is the actual unique identifier the platform uses to find and link to that specific account.
The entire OG-username market is about the @handle, not the display name — anyone can set their display name to whatever they want for free. What can't be freely set, and what has to be bought once the short ones are gone, is the actual unique username.
The username (the @handle) — it's the unique, scarce identifier. Display names are unrestricted and free to change, so they carry no scarcity value.
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.