My client wants a shopping cart for a pool part business. This will involve massive product lines. What is the best shopping cart to use, keeping in mind that i will have to manage it, need something easy to learn and manage.

"massive product lines"

I suggest an industrial strength shopping cart such as Miva Merchant. There's a new version due out in a few weeks with a whole lot of improved features e.g. search engine friendly links and the ability to do all of your page templating in Dreamweaver and export/import directly from DW to MM.

Extensive functionality is offered right out of the box but there are also a large number of inexpensive 3rd party modules that offer a huge range of customized functionality.

The areas where you should pay close attention are merchandising features (remember, your client just doesn't need a functional store - they need a store where sales and profits can be maximized), bulk upload/download of product and category data, whether the shipping and payment systems available meet your clients' requirements, if your client needs inventory tracking that the systems in the cart or via 3rd party module meet your clients' requirements and that the cart is highly extensible.

Miva Merchant can cater for huge numbers of products - I know stores which have more than 400,000 products and I know there are stores with much more than that.

There are a large number of developers (e.g. me) available for store development, development of custom applications or just consulting, on a paid basis. In addition, there is a large and very active user group for fast and comprehensive free assistance plus (now) excellent customer support from Miva Merchant itself.