You just create an account and start trading. AnandTech's forum requires you to have an account to even view the FS/FT forum, but HardForum does not. However, until you have 50 posts in other forums @ HardForum, you won't be able to post in the FS/FT forum at all. You can still PM people though (which is what I had to do when I was starting).
Generally you need some sort of references (eBay feedback, heatware feedback, etc.), and you may need to jump through a hoop or two before somebody is willing to finalize a deal with you (some people are happy with just an email to confirm an eBay account, others call you on the phone to talk to you to make sure you're not a scammer).
Since you're buying an item, it'll probably be easier for you even though you may not have a ton of references. If you were selling, very reputable traders would ask you to ship first since a decent number of "new" traders will run off with the money. As long as you represent yourself professionally, you should have any issues.
Most common methods of payment are Paypal and Amazon Payments. http://hardforum.com/forumdisplay.php?f=17http://forums.anandtech.com/forumdisplay.php?f=19