I suppose you could do this fairly easily by adding a few things to your "single.php" file.

First, you'll add a checkbox and the text regarding your terms (or a link to a popup about them)

Second, you'll add a javascript validation function that tests whether or not the box was checked.

Lastly, you'll add a call to that validation function in your "submit" field button code.  (for the Buy Now button, or the Add to cart buttons — whichever you're using)

So, when your user clicks the "submit" button, it'll run the function to check if the box has been checked.  If it has, it'll proceed.  If it hasn't, it'll show some sort of alert and stop the submit from taking place.

It's real easy for someone who knows coding if you had to outsource it.  In fact, you could probably find some validation function code if you did a Google search.