> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sumvin.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Buy and sell

> Move a user between fiat and crypto with a hosted ramp session.

<Snippet file="product-term-disambiguation.mdx" />

Buy and sell are two directions on the same **Atomic Money** ramp primitive. You call `POST /v0/ramp/sessions` — Sumvin returns a hosted widget URL the user opens to complete payment, and the resulting transaction lands on `/v0/ramp/transactions` with `transaction_type` set to `CRYPTO_PURCHASE` (buy) or `CRYPTO_SELL` (sell).

The user stays inside a single widget for payment capture, any step-up authentication, and on-chain delivery. You observe the outcome by polling the transaction.

The [on-ramp quickstart](/guides/quickstarts/onramp-meld) walks the full buy flow — quote, create session, poll for settlement — with code samples. Everything here applies to both directions; sell is the same endpoints with the source and destination swapped.

## When to use each direction

You always send `from_asset`, `to_asset`, and `chain_id`. Sumvin reads which side is fiat and which is crypto, then sets `transaction_type` accordingly.

| Direction | `from_asset`                       | `to_asset`                         | `chain_id`                                                     | `transaction_type` |
| --------- | ---------------------------------- | ---------------------------------- | -------------------------------------------------------------- | ------------------ |
| Buy       | Fiat (e.g. `USD`, `EUR`)           | Crypto symbol (e.g. `SEI`, `USDC`) | Chain of the crypto leg (e.g. `1329` for Sei, `8453` for Base) | `CRYPTO_PURCHASE`  |
| Sell      | Crypto symbol (e.g. `SEI`, `USDC`) | Fiat (e.g. `USD`, `EUR`)           | Chain of the crypto leg                                        | `CRYPTO_SELL`      |

`chain_id` always describes the crypto leg — the network you are settling crypto on (buys) or debiting from (sells). Discover the assets and chains your integration can use with `GET /v0/assets`, and check the rampable subset under [Supported values](/products/ramps/supported-values#assets).

<Note>
  Some quotes and transactions surface with `is_swap: true`. That means the upstream provider fulfilled the route by stitching a fiat ramp with a downstream swap rather than a direct route — it widens coverage for pairs no single provider lists end-to-end. The swap is fully managed upstream; `from_asset`, `to_asset`, and the fee fields on the response are still end-to-end. No extra integration work is required — surface `is_swap` to the user only if you want to explain why a particular pair is available.
</Note>

Sell requires the user to hold enough of the source crypto in the wallet you debit, and to have a fiat destination the ramp flow can pay out to — typically a linked bank account. See [Off-ramp](/products/ramps/off-ramp) for the crypto-to-fiat specifics and [Bank linking](/products/ramps/bank-linking) for account setup.

## Live coverage

The accepted values for `from_asset`, `to_asset`, `chain_id`, `country_code`, and `payment_method_type` are documented in [Supported values](/products/ramps/supported-values) — invalid inputs are rejected at the API boundary with `422 Unprocessable Entity` (enum failure) or `400` with one of `RAMP_INVALID_PAIR` / `RAMP_UNKNOWN_ASSET` / `RAMP_UNSUPPORTED_ASSET` (pair shape or coverage failure).

Format-valid pairs still depend on live provider coverage, which changes as service providers adjust their own. Treat `GET /v0/ramp/quotes` as the source of truth for what a user can actually transact — an empty `quotes` array means no provider covers that pair at that amount right now.

Payment method coverage differs by direction:

* **Buy** — commonly `CREDIT_DEBIT_CARD`, `APPLE_PAY`, `GOOGLE_PAY`, `ACH`, `SEPA`, `BANK_TRANSFER`.
* **Sell / off-ramp** — commonly `ACH`, `SEPA`, `BANK_TRANSFER`, `LOCAL_BANK_TRANSFER`. Card rails generally do not support payouts.

Pass `payment_method_type` on the quote request to narrow the list to one rail. Omit it to let Sumvin return the ranked set.

## Build the flow

Start with the [on-ramp quickstart](/guides/quickstarts/onramp-meld) — it covers the three calls (`GET /v0/ramp/quotes`, `POST /v0/ramp/sessions`, `GET /v0/ramp/transactions`), the `status` lifecycle, and common failure modes. Swap `from_asset` and `to_asset` on the quote and session request to run the sell direction; `chain_id` stays the same because the crypto leg lives on the same network either way.

## Related

* [On-ramp quickstart](/guides/quickstarts/onramp-meld) — end-to-end in 10 minutes
* [Off-ramp](/products/ramps/off-ramp) — crypto-to-fiat specifics
* [KYC passthrough](/products/ramps/kyc-passthrough) — why the widget does not re-verify the user
* [Reference](/products/ramps/reference) — status codes and error codes