$99 is steep for a lot of these domain names. You also don't disclose who the registrar is. Personally I wouldn't consider this without knowing you won't put me in GoDaddy's grasp or an equally horrible registrar.
Also it seems like you misunderstand why such a site deserves a large margin: good domains.
I don't care about paying $99 for a domain that expires in 4 mins, I could nab that myself.
What I would pay $99 for is a really great domain name, expiring... who cares because that's what your site is for.
You can always transfer the domain to another registrar. I got mine though Google for the free Google Apps account (no longer available AFAIK), and that ended up having GoDaddy be the registrar. I spent maybe 5 minutes and started the transfer process to a registrar I like better, and within a week (may have been 24 hours, I can't remember), the process was done.
As for services like this, I think they're mostly useful for domain arbitrage. I certainly wouldn't bother.
Also it seems like you misunderstand why such a site deserves a large margin: good domains.
I don't care about paying $99 for a domain that expires in 4 mins, I could nab that myself.
What I would pay $99 for is a really great domain name, expiring... who cares because that's what your site is for.