After 4-days if they haven't paid we open a UID case against them. Normally send a gentle reminder on day-3.

After a further 4-days if they still haven'y paid we close the case.

They get an unpaid strike against their account.

We have our blocks in place to block buyers with 2 or more unpaid strikes in 12-months.

We just move on after that and don't lose any sleep trying to figure out why they do it.

Just use the systems already in place. Its all you can do really.