Payment is due in calendar days, not business days. Generally you have 4 calendar days to pay before the seller can file an unpaid item dispute against you and then another 4 calendars, they can close the dispute and issue you an unpaid item strike. That's a total of 8 calendar days. Payments can be done 24 hours a day, 7 days a week. That is why you get calendar days not business days like with shipping.
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Non payers should get unpaid item strikes and be added to block bidders lists. Period.