I'm pretty much a newbie to BB as well, but I had a similar problem. I too purchased my BB via eBay and was shipped a 4.1 OS/Applications. I spoke with RIM tech support and this was the following steps I was walked through to update my BB 8700g.
1. Exit the Desktop Manager
2. Go to BlackBerry - Update your Device Software
(Must use IE)
3. Plug in your BB via USB to your PC/Mac
4. Click the "Check for Updates" button
From there you should see a list of applications that need updating, and should be able to update through the website, instead of the Desktop Manager. I'm online at the moment, so if you have trouble with that, just hit a quick reply.