NextJs: Error Obtaining eBay OAuth Access Token on Token Exchange Request

[sellmate360]

I'm working on integrating eBay OAuth into my application for user authentication. I've successfully navigated the first part of the OAuth flow, obtaining an authorization code from eBay after user authorization. However, I'm facing an issue when trying to exchange this authorization code for an access token.

Process Followed:

1. Redirected the user to eBay for authorization and received an authorization code.
2. Attempted to exchange the authorization code for an access token.

Despite following the documented steps for the POST request to eBay's token exchange endpoint, I encounter the following error:
Error: 401 Unauthorized

Outline of My POST Request:

• URL: eBay's token exchange endpoint (sandbox environment)
• Headers:
  • Content-Type: application/x-www-form-urlencoded
  • Authorization: Basic Auth with my client_id and client_secret
• Body (x-www-form-urlencoded):
  • grant_type: authorization_code
  • code: [The authorization code I received]
  • redirect_uri: [My registered redirect URI]

Checks Performed:

• The redirect_uri is exactly the same as what was used in the authorization request and is registered with my eBay app.
• The authorization code is used immediately to ensure it's not expired.
• The client ID and secret are correctly used for the Basic Auth header.

Despite these precautions, the token exchange fails. I'm looking for insights or suggestions on what might be going wrong. Any help would be greatly appreciated.

Interestingly, when attempting the authorization process for a second time with a new authorization code, the request for an access token succeeds, and I receive a valid token from eBay. This behavior is consistent: the first attempt to exchange an authorization code for an access token fails, but subsequent attempts with new codes succeed. I'm puzzled by this inconsistency and unsure why the process only works on subsequent attempts. Any insights into this behavior or suggestions on areas to investigate further would be highly appreciated. If there are specific logs or debugging strategies that could help uncover more details about the initial failure, those recommendations would also be welcome.

Callback.js:

import axios from 'axios';
import qs from 'qs';

export default async function handler(req, res) {
  if (req.method === 'GET') {
    try {
      const code = req.query.code;

      // Obfuscated credentials for demonstration
      const clientID = 'YOUR_CLIENT_ID';
      const clientSecret = 'YOUR_CLIENT_SECRET';
      const credentials = Buffer.from(`${clientID}:${clientSecret}`).toString('base64');
      const authorizationHeader = `Basic ${credentials}`;

      const response = await axios({
        url: "https://api.sandbox.ebay.com/identity/v1/oauth2/token", // Sandbox URL
        method: "post",
        headers: {
          "Content-Type": "application/x-www-form-urlencoded",
          Authorization: authorizationHeader,
        },
        data: qs.stringify({
          grant_type: "authorization_code",
          code: code,
          redirect_uri: "YOUR_REDIRECT_URI", 
        })
      });

      res.status(200).json(response.data);
    } catch (err) {
      console.error(err);
      res.status(500).json({ error: 'Error exchanging code for token' });
    }
  } else {
    res.status(405).json({ error: 'Method not allowed' });
  }
}