Market Place Cart "empty" at checkout

I cannot get the MarketPlace cart to work correctly using test products.

(1) When I select a product then click "add to cart", the widget updates. But when I click "checkout" I get the message that the cart is empty.

(2) If instead I click a second product, then the second product overwrites the first product in the widget cart. i.e. the widget cart will only display the name, price, and total of the last item selected rather than a list and running total of all items selected.

(3) If I change the cart presentation settings to "buy now", it takes me and the selected product to the next step in the check out process.

I see in similar posts that it may be a cookie problem. I checked my wp-config, .htaccess, and sunrise files and the only reference to cookies that I can find is in the sunrise file. (see below)

I've tried turning off all the plugins except domain mapping and MP. I've cleared my caches, toggling my permalinks settings, using different browsers, using a logged out browser, etc., but to no avail.

Here's my site info if someone wants to take a look:

Sales Site:
WPMU Server: Yes on
Theme: SimplePress
Active Plugins: MP, DomainMapping, WPMUDev Dashboard
SSL: Wildcat Cert. for WPMU site. SSL not required as I'm using PayPal Express Checkout.
MP activated sitewide?: No
Permalinks Setting: /%Postname%/



From sunrise.php:

if(defined('COOKIE_DOMAIN')) {
define('COOKIE_DOMAIN_ERROR', true);