In your case, if you had renewed the domain before accepting the offer, would the 15% commission have applied?
No idea, and that's the problem. There is absolutely no indication on GD what your commission rate is going to be on a given transaction.
And it's not even just renewing a domain or making sure it's working, what if a registrar has a DNS issue (like WHC did recently) and GD decides to run their check then? You could do everything right and due a DNS outage or issue, your transaction would be set at 25% and you would only find out when the payment is sent.
On Sedo the commission is right on the transaction, at Afternic you have easy access to a broker, and even at Dan, you can see on the invoice what your commission is and potentially get an error fixed before payment - or you could just ask one of the brokers. Dan also warns you to check to make sure your nameservers are working before the invoice is produced.
At GD, you have no idea what the commission has been set at, there is no chat or line of communications, and at best, you'd need to email auction support and maybe get a reply in a week's time. Maybe.
If I had known the commission on this GD sale was going to be 25% then I would have a better idea on whether to accept the offer or not, or I might want to increase it to make up for the +10%. Or just refuse it, and then start over. After all, 10% can be a significant amount on a nice sale, and not knowing (and not being able to easily find out) what the commission is at GD is a real bitch.
I swear, even when it looks like good news (a sale) it turns out to be bad news (a 25% commission) for me again, and I really need a break from this crap, even if it means $0 in May.