Setting a credit limit isn't easy or fool-proof, as Greg was saying in describing NameWinner's system. Part of it knowing who your clients are...the more you know about them, the fewer the hoops they have to jump through. A newbie domainer from Swaziland should have to jump through more hoops than GregR.
If one wants to approximate NameWinner's system, which is multi-level, quickly, one could have different levels of certification, e.g. "Platinum" to be able to bid more than $5,000, with a $1,000 safety deposit (paid by WIRE) kept on hand by eNom as collateral or a security deposit.
A "Gold" bidder, able to bid more than say $1000, would need to keep $250 as a safety deposit, and so on.
If you combine the above with a "track-record" based system, for those who you already know well (e.g. can verify bidders via telephone, or faxed contracts, or you know them from their reputations over the years and that they own lots of income-producing domains, etc.) then that might reduce the nuisance on those with stellar reputations.
Those who think they're too good to jump through any hoops, and refuse to do anything --- be firm and say "You're the weakest link -- goodbye". That just means more domains at better prices for the rest of us who are honest. Once one place has the most honest marketplace, they'll attract even higher bids overall for their registrars.
That's the main reason you see high prices on drops, vs. auctions at other places like Afternic or Sedo. On a freshly deleted name, in the perfect scenario, the bidders know they're getting a domain with perfect provenance, and that this is their ONLY opportunity to get it. Period. It's that time pressure, that they know this is IT, that makes folks reveal their true valuations.
But, if they suspect "Oh, this is probably a fake auction", or "The shills are really pushing it up....", or "Hmmmm, I wonder if the other people are really going to pay", they'll think "I better lay off...I might have another chance to get it cheaper next week", and this reduces the pressure on them to buy. Making an honest market will thus raise prices for eNom.