I have a client that purchased this theme to sell gift baskets. She wants customers to be able to select multiple items that will go into the basket. Such as different types of candy bars. This would work best as a product list with multi-select radio buttons. How can I add this to the Market Product Details so she can easily add multiple options.

When you said multiple items, did you mean more than one item at a time or one choice with multiple options to choose from?

Technically, radio buttons would only allow you to select one item at a time. The best thing to do is to add checkboxes, wherein it enables the customer to choose multiple items at a time. 

Here's an example:
<input type="checkbox" name="item" value="item1" /> Item 1
<input type="checkbox" name="item" value="item2" /> Item 2
<input type="checkbox" name="item" value="item3" /> Item 3

They should have input type of checkbox. All checkboxes should have the same name, but different values.

yes exactly …. if you wana allow the customer to select only one item at a time, then you have to use radio buttons.