Well it's hard to be very specific without you having to reveal your business idea... but heres a few tips...
> I assume you are going to make money by charging people to list ad's - much like Daft.ie or Carzone.ie.... so in the launch period perhaps give away the first 100 or 1,000 standard ad's for free to build up the site. If you don't want to do so for free, perhaps offer steep reductions (-75%)
> Advertise your website on Google Adsense - for €500 you could probably drive in excess of 1,000+ quality visitors to your website.
> Start a newsletter; by building up a database of visitors emails by issueing a regular newsletter you can retain your audience over time and help make your initial advertising campaign more effective.
> Signature... most boards/message forums allow you put put a link in your signature after every post; this can generate a fair trickle of traffic if your an active poster on message boards (Note: You CANNOT do this on AAM as it violates the TOS)
> Links - get links to your site quickly - the more quality links the better. If for example your plan is to host ad's for collectable coins - then email collectors websites and request a link in their links section... more links will boost your ranking on google.
> And the best of advice of all... offer a unique product - not another "me too" service - if your offering is any good, then it will succeed.
Hope these general tips help.
Good luck!