It depends on so many factors, there are so many pros/cons, etc.
Does your current host offer a shopping cart and security certificate?
Do you have a merchant account?
What is your budget for this?
Do you need to maintain your look and feel throughout or is it OK to have a generic checkout?
In general, the more automated it is (point and click, DIY), the more expensive it is to operate. The ones that require more initial investment (hiring a development company) are generally cheaper per transaction.
As Nikole stated... Those are some of the requirements for you to have a shopping cart. Let me get a little more in detail with what she just stated.
As for the SSL Certificate- Well it`s going to depend on two things do you have an exclusive IP or a shared IP, in order for you to install the SSL certificate you must have an exclusive IP.
My suggestion would not to go with either volusion or Yahoo Shop. They are tedious to maintain and you are limited to features and modules.
Hosting a shopping cart with volusion or Yahoo means moving your web site to their server, which I would not advise. (customer service stinks)
You can go with third party software, have it installed on your server and then it`s just a matter of linking to the shopping cart. Some third party applications can be modified to have the look and feel of your current web site and other`s well you are stuck with a generic look.
Web Developer / Owner / Consultant
When starting your own business the need to "know-how" is greater than money!