I realize this thread is a month and a half old, but I was looking through DNForum for some perspectives on GoDaddy DomainBuy; so I may as well add my latest experience.
In my case, the opening offer was $100 on a domain with only subjective value that I hand-registered within the last year. Immediately I was curious about this "Domain Buy" service, and I assumed that any buyer lazy or inexperienced enough to pay GoDaddy $50 to negotiate on their behalf must have a budget bigger than $100 for the domain in question. Naturally, I could have been wrong. But 2 weeks after my $5k offer, I received a counteroffer of $600.
So the question I find myself debating is whether $600 represents the buyer's maximum or not. If the buyer is simply an average naive person who fell for GoDaddy's "Domain Buy" service, then $600 might be as much as they've got. But I can also imagine that a large company would ask one of its employees to go get a particular domain name, and that employee will have no incentive to avoid extra costs (like GoDaddy's "Domain Buy" fee) and no incentive to do extra work (like contacting me through Whois and negotiating a price directly). And if this latter scenario is accurate, then the company's budget could exceed $600.
I've done a little digging, and I can't figure out who my buyer is. The domain corresponds to plenty of book and movie titles over the years; but novelists and publishers have negligible budgets, and Hollywood is liable to just tag on "themovie" in front of the .com to avoid paying for thousands of premium-priced domains over the years. So, if one of these guys is my buyer, I doubt I can push the price very far. The only other connection is to a World of Warcraft quest (which I just learned through Google, not being into that myself). They might have a bigger budget, but the WOW reference is several years old.
The weirdest thing about GoDaddy's "Domain Buy" service, though, is the pricing incentive for the broker. Unless I misread something, GoDaddy charges something like a $50 flat fee and 10% of the final sales price. This would incentivize the broker / GoDaddy employee to go straight for the buyer's maximum budget. But instead they opened with $100. Odd.