var users = context.Users
                .Where(u => u.ProductsSold.Any(ps => ps.Buyer != null))
                .OrderByDescending(u => u.ProductsSold.Any(ps => ps.Buyer != null))
                .Select(u => new
                {
                    firstName = u.FirstName,
                    lastName = u.LastName,
                    age = u.Age,
                    count = u.ProductsSold.Count(p => p.Buyer != null),
                    products = u.ProductsSold
                    .Where(ps => ps.Buyer != null)
                    .Select(ps => new
                    {
                        name = ps.Name,
                        price = ps.Price
                    }).ToList()
                }).ToList();

            var json = JsonConvert.SerializeObject(users, Formatting.Indented);

            return json;