The main "punishment" is that sellers can choose to set a block for anyone with at least 2 unpaid item strikes (or whatever the corporate-speak translators are calling it since they revamped the process this Spring) in the past up to 12 months. But not all sellers have enabled that block and not all who have enabled it have enabled the strictest settings (sellers can set more than 2 strikes needed, or can choose 1 or 6 months instead of 12--and 6 months was the maximum years ago so older sellers who set it back then might still have that lookback period and 1 month was the default in the box for that field so a seller who didn't change the defaults would block anyone with 2 in the past 1 month).
But most sellers who know what they are doing do set it at 2 in 12 months, so it will no longer apply to you 12 months after your second to last strike has its first anniversary.
Other than that eBay considers unpaid item strikes in setting Buyer Activity and Open Transaction Limits (and counts them towards the latter for an unknown period of time; how long they go back in determining the limits is also not public knowledge). And eBay can suspend or permanently kick off a buyer for non-payment if s/he gets too many (no idea how many that is in how long a period) or in conjunction with other policy violations.
