What is the use for that when sellers use the buy it now option? Won't stop them buying. Even if a seller does check after they have bought,  it's too late then. They've already bought. They could have 1, 000 orange dots and it would make no difference. 

Best way to stop them is have blocks in place. Ebay won't stop them regardless of how many times they've not paid. That has been proven countless times. Therefore, it's up to the seller to protect themselves as best they can.