I've never had any of my names accepted for Great Domains but lately things have changed as the minimum bid now seems to be $100 for Great Domains and not $60. Commissionwise I understand it's 20%, I could be wrong...but I'd be very surprised if it was the standard 15% since they spend so much advertising Great Domain Auctions months ahead of time, that ain't cheap...
But no, they don't charge you a higher commission if you don't park with them. I stopped parking w/them long ago but I sell (and sometimes even buy) thru them and commissions are the same for sellers, 15% if an internal Sedo deal with a minimum of $50 (whichever is greater), and 20% if the buyer was procured thru their MLS system or their agreement w/GD, ie the offer for the Sedo listing was made from another marketplace, like Afternic or GD. The $50 min commission the seller will pay applies here too...
Biggie may be more attuned to answer this since he and other pros here enjoy SedoPro status, I don't know if different commissions (lower) apply to them in certain cases, maybe less restrictions ?.... and priority may be given for domain submission for their Great Domain Auctions, those could be some of the perks the SedoPro crowd may be gettin there. I've heard all 3, but do not know for a fact it's true...
It's been a long time since I last ran any sedo auctions. Have a good few running in their latest auction greatdomains auction. Do they still charge a higher cut if you don't park the name with them and they sell via their auctions?
