Cart Checkout Error MarketPress

I've read many of the threads here about this issue and I have extensive troubleshooting but still having this problem:

Everytime I try to test the checkout, I get this error message:

There was a problem connecting to PayPal to setup your purchase. Please try again.

10002 - Security error - Security header is not valid

Here's info about my site in development:

I am using MarketPress for a multisite
I set up 3 different Sandbox accounts through PayPal.
1 for the main site
2 others for vendor sites

I put the Sandbox API Credentials for the main site in the Network Admin > Settings > MarketPress for the PayPal Gateway Global Cart.

I put the Sandbox e-mail addresses in each Site's Store Settings in the Payment area where it only has one form field that says "PayPal Merchant E-Mail"

I've checked for extra spaces, redone this several times, etc. and I am still getting the error message.

What am I doing wrong!?!?!?!?!?

Thank you very much for all your help.