fix: show actionable Business Policy setup instructions instead of raw JSON on 400 error

EbayListingTool/Services/EbayListingService.cs

@@ -1,4 +1,4 @@
-using System.Net.Http;
+using System.Net.Http;
 using System.Net.Http.Headers;
 using System.Text;
 using EbayListingTool.Models;
@@ -174,6 +174,35 @@ public class EbayListingService
         }
     }
 
+    /// <summary>Parses an eBay error JSON body into a user-friendly message.</summary>
+    private static string ExtractEbayError(string json, string policyType)
+    {
+        try
+        {
+            var errors = JObject.Parse(json)["errors"] as JArray;
+            var first  = errors?.FirstOrDefault() as JObject;
+            if (first != null)
+            {
+                var errorId = first["errorId"]?.Value<int>() ?? 0;
+                var longMsg = first["longMessage"]?.ToString() ?? first["message"]?.ToString() ?? "";
+
+                // 20403 = account not opted in to Business Policies
+                if (errorId == 20403 || longMsg.Contains("not eligible for Business Policy"))
+                    return $"Your eBay account is not set up for Business Policies, which are required to post listings via the API.\n\n" +
+                           $"To fix this:\n" +
+                           $"1. Log in to the eBay Seller Hub (or sandbox Seller Hub)\n" +
+                           $"2. Go to Account \u2192 Business policies\n" +
+                           $"3. Create at least one Postage, Payment and Returns policy\n\n" +
+                           $"Once done, click Post to eBay again.";
+
+                if (!string.IsNullOrWhiteSpace(longMsg))
+                    return $"eBay {policyType} error: {longMsg}";
+            }
+        }
+        catch { }
+        return $"Could not fetch {policyType} from eBay. Please check your account settings.";
+    }
+
     private async Task CreateMerchantLocationAsync(string token, string postcode)
     {
         if (string.IsNullOrWhiteSpace(postcode))