using static System.Console;

using System.Collections.Generic;

using System.Linq;

public class Program {

    public static void Main() {

        var lista = CriarCompras(10);

        lista[0].Item = 1;

        foreach (var item in lista) WriteLine(item.Item);

    }

    private static List<Compra> CriarCompras(int numComprasParaGerar) => new int[numComprasParaGerar].Select(i => new Compra()).ToList();

}

​

public class Compra {

    public int Item = 0;

}