To get more people interested in your computer, you should supply as much information as possible about it. People like to know exactly what they are buying, and if that information is not available, they are not likely to place a bid. There are many ways of finding out what the hardware and software specifics are for your computer. The hard way is to open up your case and write down the part numbers for all the important things people like to know about, like RAM size and speed, processor model and speed, etc. You can then look up those numbers online to find out more about them. This is rather time consuming, and could be dangerous as well. Many components inside your computer can be damaged by static electricity, so grounding yourself properly is a must. I suggest that you do not take this approach. Most of the important information that people really want to know can be obtained rather easily by right clicking on 'my computer'. This will show you your processor speed. It will show you how much RAM is installed. It will show you what operating system you are running, and it will show you what version it is.
If you want more detailed information, like what additional software is installed, and more specific hardware specs, you can download a program like Belarc Advisor, which will tell you more than you ever wanted to know.... like the model numbers of your hard drive and motherboard, installed programs and license information... the only problem is that Belarc Advisor will not show you non-windows partitions. This will not be a problem for most people. If your computer is a multi-boot machine that uses linux as one of its operating systems, then Belarc will not give you any information about that partition.... but if you are computer savvy enough that you have a multi-boot computer, then you probably don't need any third party software to tell you about your PC anyways.

Always be aware that there are scammers out there who will want you to ship to Nigeria or some other scammer's paradise by fooling you into thinking that you have already been paid. Don't fall for anything. If this becomes a problem for you, then your best bet is to determine what price you want for your computer, list it as fixed price 'buy it now' and check the box labeled 'require immediate payment'. Be sure to say in your listing that you will ship to confirmed addresses only, set your preferences to block bidders from countries to which you do not ship, and block unconfirmed address payments in your paypal preferences. As always, use delivery confirmation for payments up to $249, and signature confirmation for payments $250 and over.