Ok, here goes:
On the design front - as this is a new site and probably a new concept for some, I would spend at least half your home page real estate explaining with some very simple graphics what the site does and how. Don't make the assumption that people understand how it works. You have sort of a story in the 'about' section but you need to reshuffle this. Look at 37signals product landing pages for inspiration. The 'surprise me' links are a good idea, I'd just have these on the front page and remove the regular stuff like old monitors.
Your product listing should have clickable images/headers. People expect to make a mouse click anywhere in the general vicinity of a thumbnail and get to the detail of that image.
You've used the term 'ditched time' - but it's not clear what this means. Does this mean the item was available up until this time, or does it mean it is available from this time onwards? I'm assuming that for the person disposing the goods, there is a set time limit for how long they are goign to hang onto stuff before it goes in the tip or charity bin. So you should have an 'available from' and 'final ditch date' or something similar - a window of time in order for the person who wants it to take action - even better would be one day to entice the person collecting to commit to doing it. Short timeframes encourage action. A 'time window' in terms of a clock or bar graph would be a good reminder of this.
You might also want to put in some geo-location so people can enter their postcode and find stuff near to them. It's not that complicated as the geo-location for postcodes data is freely available and not very volatile. It's not difficult to write a basic algorithm that calculates this for you.
Finally, and I'm not sure if you have this or not, but your main problem is going to be stale inventory. You'll need to be aggressive in finding ways of removing old inventory from the site, whether that be in a voting system or aggressive removal of items after x days. If potential users continually find that items they want are already gone they will give up on the site.
For promotion you should do some guerilla action on places where cash strapped people are. I'm thinking university campuses. A good idea would be to get a heap of old junk like pots, pans, couches, wardrobes - stuff which is useful but not desired. Spray paint your logo with a simple template and dump it where people will find it. Something like 'find free stuff with ditchit.com.au'. With any luck you'll get in trouble with the authorities and have a good story to hit the media with and generate your own PR. You could also do this with a roadside dumping of stuff if you think you can locate a good place for it. Think of the sorts of places people park their cars with 'for sale' signs in them.
Alternatively, pro-actively list someone's stuff on the site and then organise users to come around and clean it out, then take the 'story' to a local newspaper (the free type will do). They're desperate for content so you'll have no trouble getting in. Use the 'residents helping residents' angle and they'll eat it up. People who read free local newspapers are bang on your target market for both disposers and collectors of unwanted stuff.
You might also allocate a few hundred bucks for garage sale purchases, and go around and build up some inventory to move through a variety of planted user accounts. Much cheaper promotion than anything else because you need the network effect to kick off your site.
Good luck and congrats for getting to launch.