Paypal do not immediately reverse the payment - the seller is given some time (14 days I think) to resolve the matter - if the buyer is still not happy then they can proceed with the dispute.
I lodged a dispute once because the item had not been received from a Chinese based ebay seller - my money was refunded with 24 hours :)
The theory behind the proposal is good, but I think the practicalities may be somewhat more difficult. With Group Buys we are trying to use our collective bargaining power to achieve reduced prices on products, or to share in the, sometimes, high postage costs. The majority of GB's I have been involved in have been with an overseas retailer so the escrow idea would not work. For locally based businesses, I imagine that the profit margin on the GB's would be slim so having the cash in the bank when the order is placed is probably the only benefit they get out of the transaction.