15% or even 20% commission sounds fine if their brokers are going to spend a serious amount of time prospecting likely buyers and then hit the phones and talk to the right people to make a deal at least possible. But for taking a walk-in lead off a parking/sales lander (or even a lead via the internal Sedo search engine) it sounds very greedy indeed.
For a bit of perspective, think about all the work a good estate agent puts in before they see their 0.5% or 1% commission. Sure, it's on a (likely) higher-ticket item, but they still do a whole lot of real work for their cut!
And if you're paying 1% commission to an estate agent on the sale of a £250,000 house, their take is £2,500. That's the same commission as a domain "broker" would take on the £12,500 sale of a domain if the commission rate's 20%. And (99% of the time, it looks like) for much, much less effort!
I am not suggesting that Sedo should proactively sell all the millions of names on their platform. But there should be two very different commission tiers, e.g. 5% and 20% - to earn the 20% they would have to find a buyer within (say) the first 60 days of listing and at a level that meets your pre-set price expectations.