Because Snapchat makes it harder to change a username after the fact compared to most platforms, it's worth being extra clear on the handoff details before paying — you want to know exactly how login/recovery access transfers.
Once the deal is done, immediately update the account's own recovery details (see how to protect a username after buying) — this matters more on Snapchat than on platforms that make it trivial to re-secure an account.
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.