If you don't want this to turn into a marketplace for squatters, you should really set some ground rules. For example, you may say that people can't post domain names that they're selling for more than $100. That's just one thing, there are probably others that could help this website not turn into a trash collector.
That kind of policy only helps the squatters. If you set a $100 maximum then you'd just get squatters buying them all for $100 and reselling them for much, much more. There's not much difference in paying $100 for something and paying $10 for something, if you're going to sell it for $10,000 or $100,000.
Anyways, about the idea, one of two outcomes is possible:
1. If they domain is worth more than threshold, someone will buy it and take advantage. I don't see a difference of a startup benefiting versus someone else reselling it. the difference in value is still being captured.
2. Garbage names, nobody cares and the site dies.
I know I am not the only domain investor reading HN and if there is quality, someone will buy them.