Hi Horst,
By default, no checkboxes are required — so those would have to be integrated into the shopping cart checkout area.
I don't see it being that difficult to add a couple of checkboxes with text links to the documents into the shopping cart page. (the page that displays is "ajCart/cart.php") You could then just setup some javascript alerts that popup and cancel the post if the checkboxes are not checked.
To have this actually "recorded" may be a little tougher, because Market Theme connects with Paypal. All of the customer information (name, address, email, products ordered, etc.) is archived by Paypal — and not your WordPress Market Theme install.
You could certainly not let them proceed until they check the boxes, but for Paypal to record them — it would require some research into how to provide additional custom data. Pretty sure it could be done though.
I hope this helps.