Add API route for selling shares, document

docs/docs/api.md

@@ -579,6 +579,26 @@ $ curl https://manifold.markets/api/v0/market/{marketId}/resolve -X POST \
                  ]}'
 ```
 
+### `POST /v0/market/[marketId]/sell`
+
+Sells some quantity of shares in a market on behalf of the authorized user.
+
+Parameters:
+
+- `outcome`: Required. One of `YES`, `NO`, or a `number` indicating the numeric
+  bucket ID, depending on the market type.
+- `shares`: Optional. The amount of shares to sell of the outcome given
+  above. If not provided, all the shares you own will be sold.
+
+Example request:
+
+```
+$ curl https://manifold.markets/api/v0/market/{marketId}/sell -X POST \
+    -H 'Content-Type: application/json' \
+    -H 'Authorization: Key {...}' \
+    --data-raw '{"outcome": "YES", "shares": 10}'
+```
+
 ### `GET /v0/bets`
 
 Gets a list of bets, ordered by creation date descending.

web/pages/api/v0/market/[id]/sell.ts

@@ -0,0 +1,28 @@
+import { NextApiRequest, NextApiResponse } from 'next'
+import {
+  CORS_ORIGIN_MANIFOLD,
+  CORS_ORIGIN_LOCALHOST,
+} from 'common/envs/constants'
+import { applyCorsHeaders } from 'web/lib/api/cors'
+import { fetchBackend, forwardResponse } from 'web/lib/api/proxy'
+
+export const config = { api: { bodyParser: true } }
+
+export default async function route(req: NextApiRequest, res: NextApiResponse) {
+  await applyCorsHeaders(req, res, {
+    origin: [CORS_ORIGIN_MANIFOLD, CORS_ORIGIN_LOCALHOST],
+    methods: 'POST',
+  })
+
+  const { id } = req.query
+  const contractId = id as string
+
+  if (req.body) req.body.contractId = contractId
+  try {
+    const backendRes = await fetchBackend(req, 'sellshares')
+    await forwardResponse(res, backendRes)
+  } catch (err) {
+    console.error('Error talking to cloud function: ', err)
+    res.status(500).json({ message: 'Error communicating with backend.' })
+  }
+}