public void ProductRegistration(Product productToRegister)
        {
            Seller soughtSeller = sellerList.Find(x => x.TaxNumber == productToRegister.SellerTaxNumber);

            if (soughtSeller != null)
            {
                soughtSeller.ProductList.Insert(productToRegister);
                productsList.Insert(productToRegister);
            }
        }
        public bool SellerInsert(Seller newSeller)
        {
            var seller = sellerList.Find(x => x.TaxNumber == newSeller.TaxNumber);

            if (seller == null)
            {
                sellerList.Insert(newSeller);
                return(true);
            }
            else
            {
                throw new SellerExists(newSeller as Seller);
            }
        }
        public bool BuyerInsert(Buyer newBuyer)
        {
            var buyer = buyerList.Find(x => x.TaxNumber == newBuyer.TaxNumber);

            if (buyer == null)
            {
                buyerList.Insert(newBuyer);
                return(true);
            }
            else
            {
                throw new BuyerExists(newBuyer as Buyer);
            }
        }