First, you should not figure it would only take 4-5 days to make enough posts. Also, I think there's a minimum amount of time that you need to be a member. I didn't check your stats so I do not know where you stand but I can assume you're a ways off and not just in post count.
I did a quick search for you of the Marketplace and there aren't any Power Macs 9600 for sale currently. Even if there were, there'd be that whole issue of you getting into that section to do your deal. But take comfort in knowing there aren't any sales right now.
Have you ever used If This Then That? (IFTTT) It's an app that is absolutely rad. You can set up a search in eBay and tell this app to e-mail you the moment something pops up. I have it set up for Craigslist for a specific item that has been sold second hand but rarely comes up. That app has already paid off for me...well it is free but it is useful. Check it out and put things together so you're notified. I doubt you need this machine right away, I could be wrong, so patience is something you'll need to have.
I would use eBay and Craigslist for these items and eBay will likely be your better bet.