Many sites like this already exist...
sedo flippa godaddy auctions brandbucket etc
I don't see anything different in the value proposition, what is your difference to the other sites?
Good feedback on the value prop. I will make that clearer.
> Show HN is for something you've made that other people can play with. HN users can try it out, give you feedback, and ask questions in the thread.
> If your work isn't ready for people to try out yet, please don't do a Show HN. Once it's ready, come back and do it then.
> ...sign-up pages...can't be tried out, so they can't be Show HNs.
I suggest you actually build out the application and resubmit then, or find another traffic source with which to validate the idea.