A regular username is whatever's still available when you sign up — usually a long string with numbers tacked on because the clean version is taken. An OG username is a short or clean handle that's already taken, by someone who grabbed it early, before it became scarce.
You can't tell an OG username apart from a regular one just by looking at it — @zed registered today (if it were somehow available) would look identical to an OG @zed from years ago. What makes it OG is that today's signup form won't let you have it — someone already does.
A regular username costs nothing — you type it in and it's yours. An OG username has a price precisely because the free path is closed. That scarcity, not any technical difference, is the entire reason a market for them exists at all.
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.