There would be a bunch of legalities to be considered for a system like that - think about service providers and the like, for instance. Some of them will ask new customers to pay a certain amount up front, which is then kept in reserve while they bill the customer as normal from month to month or whatever, and after a certain amount of time has passed, they either refund the amount or apply it to a current bill, but it also helps keep the account current if the buyer defaults.
But... what they don't do is forcably remove funds from the buyer's accounts*, funding sources or whatever. If a buyer defaults on a contract and owes more money than the company has received, they have to revert to the traditional methods for collection, where viable. And even then, there's quite a big difference because it becomes money owing on an item or service already provided, which doesn't happen for an eBay sale. You won't see many (if any) retail stores trying to get money if someone doesn't complete a lay-by (I do realise some stores charge a non-reversable lay-by fee to cover admin costs, which is charged up front).
I also honestly think that the fewer circumstances there are where a buyer resents a seller before the item has even been sent, the better. I understand non-payers are a PITA, and incur some non-recoverable costs, but if a seller genuinely wants a sale contract to be enforced (which is, at the very least, possible via the court system), eBay / PayPal are not the right companies to do the enforcing. As much as NPB can cause problems, eBay / PayPal don't need more power and authority over transactions.
When funds are returned to a buyer via a protection case, unless I'm mistaken in my understanding of how / why it works, it's because a contract is deemed to have not been fulfilled, rather than it being an enforcement of one (would welcome more knowedgable insight on that one).
Plus, when you read eBay's responses to issues like this, it's pretty obvious their current attitude is that if something is part of the "cost of doing business" on eBay, the sellers are the ones who know about it, but the buyers who do pay are supposed to pay for it. (I do want to mention that eBay considered a plan to reduce instances of NPB a year or two ago, whereby all items on the site were immediate payment required, and that for auctions, an authoristation was granted at the time of a bid being placed that would mean the winning bidder would have the sale amount deducted from their nominated funding source - it never became anything more than an idea being kicked around though, and I suspect for good reason - referring more specifically to the pre-authorised payment. There's a few different ways eBay could make immediate payment viable on BIN items, even when postage on multiple purchased needs revision).
*pre-arranged debit agreements notwithstanding, which wouldn't be 'forcably' removing money, anyway.
