Every now and again I check my Buyers Log in the Site Preferences and can see just how many low feedback buyers have been prevented from buying from me because they have 2 unpaid strikes.

Many of them have feedback under 5.

Before I put my strongest blocks in place I was having buyers do similar things to what you describe - buy one item, not pay and then buy different items a little later. 

Set your blocks to also cover buyers who are not phone verified and have a feedback score of less than 5.