| 402 | payment_failed | Payment or payout could not be processed | Check the method, then a new quote |
| 403 | — | Organization not allowlisted | Contact support |
| 404 | checkout_not_found, cart_not_found | Quote missing | Check quoteId or create a new quote |
| 409 | subtotal_mismatch | Live buy total went above expectedTotal, or live sell payout went below it | New quote, pass the new total |
| 409 | listing_validation_failed, no_eligible_items | Items no longer available | New quote with live listings or offers |
| 409 | listing_quantity_mismatch | Sell qty overlaps your active listings | Listing conflicts, or adjustListings / confirmListingMismatch |
| 409 | system_check_failed | Trading halted or eligibility failed | Poll GET /execution/v2/trading-status |
| 409 | seller_payment_unavailable | A listing could not be paid out | New quote |
| 409 | payment_in_progress | Confirm already has a payment in flight | Wait and retry this quote; do not double-charge |
| 410 | checkout_expired | Quote window elapsed (sometimes omitted on validation) | New quote |
| 422 | checkout_empty | Quote has no line items | New quote with at least one item |
| 422 | invalid_quantity | Quantity is missing or not a positive integer | New quote with quantity ≥ 1 |
| 422 | invalid_payment_method, payment_method_not_configured, stripe_customer_not_configured, card_payments_not_allowed | Payment setup | Use wire_transfer or a Stripe pm_ / ba_ id; complete Stripe onboarding |
| 422 | optional_fee_mismatch | Sell optional fee drifted | Refresh quote, pass updated expectedOptionalFees |
| 422 | vault_shipping_not_applied | Vault shipping was requested but did not apply | Check eligibility on optionals |
| 422 | instant_payout_insufficient_credit | Instant payout credit used up | Drop instant_payout or wait for credit |
| 422 | promotion_code_invalid, promotion_code_expired, promotion_code_not_eligible, promotion_code_exhausted | Promo rejected | Preview again, then execute without the code or with a valid one |
| 422 | payment_rollback_failed, payment_needs_reconciliation | Payment captured but the order did not complete | Contact support — do not retry |
| 500 | system_error | Unexpected checkout failure | New quote once; if it persists, contact support |