EbayListingTool/EbayListingTool/Services/EbayPriceResearchService.cs

using System.Net.Http.Headers;
using Newtonsoft.Json.Linq;

namespace EbayListingTool.Services;

public class LivePriceResult
{
    public List<decimal> Prices     { get; init; } = new();
    public int           Count      => Prices.Count;
    public decimal       Min        => Prices.Count > 0 ? Prices.Min()    : 0;
    public decimal       Max        => Prices.Count > 0 ? Prices.Max()    : 0;
    public decimal       Median     => Prices.Count > 0 ? Percentile(50)  : 0;
    public decimal       Suggested  => Prices.Count > 0 ? Percentile(40)  : 0; // 40th pct: competitive but not lowest

    private decimal Percentile(int pct)
    {
        var sorted = Prices.OrderBy(p => p).ToList();
        int idx = (int)Math.Round((pct / 100.0) * (sorted.Count - 1));
        return sorted[Math.Clamp(idx, 0, sorted.Count - 1)];
    }
}

/// <summary>
/// Uses the eBay Browse API (app-level token, no user login required) to fetch
/// current Buy It Now prices for similar items on eBay UK.
/// </summary>
public class EbayPriceResearchService
{
    private readonly EbayAuthService _auth;
    private static readonly HttpClient _http = new();

    public EbayPriceResearchService(EbayAuthService auth)
    {
        _auth = auth;
    }

    public async Task<LivePriceResult> GetLivePricesAsync(string query, int limit = 20)
    {
        if (string.IsNullOrWhiteSpace(query))
            return new LivePriceResult();

        var token   = await _auth.GetAppTokenAsync();
        var baseUrl = _auth.BaseUrl;

        using var request = new HttpRequestMessage(HttpMethod.Get,
            $"{baseUrl}/buy/browse/v1/item_summary/search" +
            $"?q={Uri.EscapeDataString(query)}" +
            $"&filter=buyingOptions%3A%7BFIXED_PRICE%7D" +
            $"&limit={limit}" +
            $"&sort=price");

        request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);
        request.Headers.Add("X-EBAY-C-MARKETPLACE-ID", "EBAY_GB");

        var response = await _http.SendAsync(request);
        var json     = await response.Content.ReadAsStringAsync();

        if (!response.IsSuccessStatusCode)
            throw new HttpRequestException($"eBay Browse API error ({(int)response.StatusCode}): {json}");

        var obj    = JObject.Parse(json);
        var items  = obj["itemSummaries"] as JArray ?? new JArray();
        var prices = new List<decimal>();

        foreach (var item in items)
        {
            var priceVal = item["price"]?["value"]?.ToString();
            var currency = item["price"]?["currency"]?.ToString();
            if (currency == "GBP" && decimal.TryParse(priceVal,
                    System.Globalization.NumberStyles.Any,
                    System.Globalization.CultureInfo.InvariantCulture,
                    out var p) && p > 0)
            {
                prices.Add(p);
            }
        }

        return new LivePriceResult { Prices = prices };
    }
}