Unless you have to use DNN I'd check out a few of the many ready-made PHP job board scripts around. For DNN, there are a couple of modules I've seen on Snowcovered that might suit - have a search there. (they should pay me for all the times I mention them!). I recall Dotnetjedi has one, though it's from DNN2 days and might be getting a little stale.. dunno. I've also heard people have used Ventrian's Property Agent to do somethign similar. It's great for creating browsable categories of information, but may not do the posting thing.

Rob