I use MarketPress Lite AND a Full version of MarketPress across several sites.
It seems PayPal is very strict when A user enters an address from the United States.
1: Fill in your shipping details with a valid address (my customer was from LA, USA) - use google to get some random valid address.
2: Enter 'LA' into the city field (instead of 'Los Angeles').
3: Click 'Continue Checkout'
You are returned to a page with only the 'Continue Checkout' button which the end user tries to resubmit.
A MarketPress/PayPal message explaining an invalid address or similar error message from PayPal displayed in the page after failure.
I have not tested this on the other address fields when using 'Country: United States' but this only seems to occur for customers from the USA so I assume there are possibly some strict address checks for USA customers at PayPal. I also assume PayPal is returning the error but it is unknown to MarketPress full/MarketPress lite (after all, it appears to work fine for other countries when you enter an invalid address).
I hope this helps the development of the full and lite versions of MarketPress (and yep, it is loads better than WP-ecommerce as stated in the description)