The problem we saw is that if you have a few domain names that you're holding onto but not using, there was no super easy way of letting the world know that you're open to receiving offers for them.
The value proposition with http://domainholder.io/ is that you can set up a super simple 'For Sale' page on any domain you own with a payment button and contact form in just 15 seconds. All you need to do is add an A record and a TXT record wherever you registered the domain name. Every domain that you 'park' with us automatically cross-promotes your other domains as well.
So far we've got a few hundred domains parked with us and people seem to be liking it thus far. If you'd like to give it a go with some of your domains we've got a free plan: http://domainholder.io