Richface, Sedo and the seller can do exactly what you mention if they want to. I've been contacted a couple of times about this in regular auctions where the winner didn't pay, once I bought the name for the same amount of the 2nd highest bid (mine) and I was also contacted once after I had bid on a Great Domain, but apparently the seller told Sedo I could only have it if I increased my offer to what he now wanted (but it was still below reserve), I didn't go for it
If a seller gets a price close to what his reserve was in Great Domain Auctions he'd better think twice before refusing to offer it to highest bidder, give the times we're in. Sedo won't list it in Great Domains for some time to come after it's already made the rounds in their ad campaigns, they prepare and advertise this months in advance. If they did that the domain would become stale, and it would work against the seller. As for Sedo brokers (or any) they're simply too expensive. It's true, the seller cannot know who the 2nd highest bidder was if the winner is a deadbeat, or who the bidder who came nearest to reserve is, but sellers can tell Sedo to contact those bidders, and Sedo should like this because it still makes its commission
But if you're the highest bidder and the domain wasn't sold because reserve wasn't met, then simply contact the seller using the WHOIS afterwards, Sedo cannot prevent this since the auction is over, neither party could get penalized. As the interesed party you'll get a better deal buying since there is no 20(or 15)% Sedo commission to be paid by the seller, so he'll give you a lower price
Of course the seller is never forced to sell below a reserve he set, or below a minimum bid/offer he wanted, and Sedo normally does NOT take the initiaitve after an auction ends and reserve isn't met, the seller needs to tell Sedo, and usually Sedo will do it, a bit less in commission is much better than no commission. If a buyer wants to buy and the seller agrees on a price Sedo will do all it can to close a deal, no question
Another suggestion is also that when reserve wasn't met the seller could then start an auction w/the highest bid he got as the new starting bid, it would be smart on Sedo's part to suggest this to sellers. It gives Sedo the assurance of a commission since a sale will take place, and the domain will make the rounds 7 more days, possibly even fetching a higher selling price, which works even better for both Sedo and sellers