What do you do with a finished product?
proexploit: How you get users depends on what your goal is. It looks like your site offers a free service and has advertising (AdWords and selling ads directly [hint: telling users to name a price is not a good idea.]) as it's only income source. This pretty much rules out paying for users unless it can be done very cheaply as you aren't going to make much per new user.SEO is good. A properly configured site can work wonders, but beyond that, a good product will "SEO itself". Spending time on linkbuilding campaigns or blackhat SEO will not be efficient in the long run.Buying ads could be expensive and wouldn't be worth it to me. That would depend on where you can advertise and the cost, but I wouldn't do it.Spam is an obvious no.So what can you do? Well, this post right here will more then likely get some visitors to check out your site (if that was your intent, +1 sneaky). I'd focus on creating the best product you can, including a better design, alternative monetization methods, and asking HN or current users for suggestions on improvement and implementing them.
[sidenote: I don't have many projects I consider finished, only paused if I'm not working on them. Stay open to future development. If you aren't adapting and improving, you're falling behind.]You could contribute to debate forums or other relevant websites, posting genuinely helpful comments relevant to your site and harmlessly introducing it. I personally like to only use advertising methods that I would not be annoyed with, but if your only goal is to make money, just spam your site all over the place. You might make a little cash, but you'll get a bad reputation doing it.