MarketPress - Help with Paypal API Madness

Been chatting to one of the guys at Paypal for a few weeks now trying to get my API credentials setup for MarketPress.

They want to test the store, but I cant seem to get things working. I found it weird that they want to test the app that is not approved yet, but I assume I need to have the sandbox setup?

So, I've applied for the app, have everything but the application ID which says it aint necessary to test.

I have all the other details filled out.

I also applied for a sandbox and have that setup. I keep on getting various errors when I click on purchase a product.

I have tried various combinations of leaving the app info out, adding just the sandbox info and tested the main site and other subblogs, but I always run into one of these:

There was a problem connecting to PayPal to setup your purchase. Please try again.
520009 - Account thxxxxxxxxt@gmail.com is restricted

There was a problem connecting to PayPal to setup your purchase. Please try again.
560029 - The required X-PAYPAL-APPLICATION-ID header is missing from the HTTP request

There was a problem connecting to PayPal to setup your purchase. Please try again.
579040 - The receivers george_128xxxxxxxx@thatmlmbeat.com and george_128xxxxxx@thatmlmbeat.com can\'t belong to the same PayPal account

While they are all self explanatory, I still don't know how to get them to work.

Anyone know what I'm missing here?

Thanks,

George