Currently, I can't offer you any custom code to offer both "buy now" and "add to cart" buttons features.  It's got to be one or the other.

The only way for you to offter both affiliate products and your own products is to enter "affiliate products" a little differently than you enter regular products that you are selling yourself.

Here's what you'd do:

1. Set your store up to use the "Add to Cart" functionality.

2. Enter products you sell yourself regularly.

3. For affiliate products, you'll need to leave the "Price" field blank, and put the product price into the beginning of the Description field.  (This will prevent the "Add to Cart" button from showing)

4. Then, you would enter the affiliate link and button (or link text) into the description field after your description text. 

(Or if you'd rather, you can enter the affiliate link code and hypertext into the "Additional Details" field.  If you do that however, you'll probably want to edit the Market Theme language file (mkt_language.php), to change the words "Additional Details" to something else, or lave blank altogether.)

This should give you the result you're after.