Because you cannot physically look at the item, make sure that you are getting what you want. Many sellers on eBay may not have very good feedback, but when it comes to buying anything here, you really want someone competent. Speaking of competent, buy a "whitebox" computer. Systems like Dell, HP, Compaq, Gateway, Toshiba, Sony, and everyone else just sell crappy systems. I personally have a Dell computer from early 2000 that is still running strong and it works great as a backup to my gaming rig, but there is difference now adays. We bought that for close to $2500 new. It is a good computer. Dell's and others now are offering $299 compete systems. How good can a $300 computer be? First they will be using onboard everything. I will get into that later, but basically it sucks off the main CPU to fuel its own needs like sound and video. Not good. Also their computers are not upgradeable. They will be obsolete within a year, and you can't just buy an upgrade, you have to buy a new one in most cases. Also I have seen too many systems like these just stop working after around 3 years when the warranty goes. Why? Because they don't use quality parts! It is that simple! Buy a whitebox and you will be much happier, and they come in prettier towers too! :)
    Many times you hear how many "Gigahertz (Ghz)" a computer has, but that is not everything. There are AMD and Intel Systems. AMD have different types, and right now you want to go with an Athlon64 or Athlon FX or Athlon FX2. Intel's come basically in 4 varieties. Celeron D is the lowest of the processors, and is good for internet surfing, basic office software like Word and Excel, and the free Windows Games such as Solitare. Then there is Pentium 4, Pentium D, and Pentium Extreme Edition. You want to look for a Pentium 4 with HT (Hyper Threading) if you do everything a Celeron buyer does, but you run more applications at once. Low end games are good for speeds up to 3Ghz, and you might be able to get away with some higher end games run at lower settings with a Pentium 4HT around 3.4Ghz-3.8Ghz. Pentium D is like having 2 computers at once so a lower speed 2.8Ghz is acceptable for the same as what a P4HT 3.8Ghz could do. The Extreme Edition chips are really fast Pentium D's. Rarely do you see these advertised because this one component can cost as much as $1200 by itself! If you see a really cheap computer with one of these be skeptical, because it might be falsely advertised. A system with a P4 EE should be atleast $800 on eBay. AMD systems equate roughly liek this: Athlon 64 => Pentium 4, Athlon FX => Pentium D, Athlon FX2 => Pentium EE. Keep in mind a lower "speed" in Ghz is acceptable with an athlon because they are just as powerful. Look at the number (AMD's are usually 2600+ or 3800+ or something inbetween) and divide by 100. That's kind of roughly similar to the "speed" measurement of a P4.
    Speed is what you want in a new computer, but Ghz alone doesn't constitute a blazing system. Often you need lots of RAM. Most common sizes are 128Mb, 256Mb, 512Mb, 1Gb (1024Mb), 2Gb (2056Mb) and any combination of 2 or 4 of these denominations. If that isn't enough already there are different speeds that they run at and these speeds have different deliniations. There is usually a number following either a PC or PC2 like PC 3200, or PC2 5300. PC xx00 is DDR memory usually. The higher the number, the faster it is. It is old technology, but still work like a charm and even companies like Dell are still using it. PC2 is DDR2 (notice the 2) meaning that it uses more pins (240 pins compared to DDR's 184) meaning more stuff can be done at once, meaning faster speeds. With RAM, more capacity is always better and the faster that memory is, the better. A Celeron or equivalent user probably wants between 256Mb and 512Mb minimum. Go for more if you can! A person who uses photo progams a lot, or watches a lot of movies on their computer while typing a report, probably wants 512Mb minimum and I recommend 1Gb. Serious gamers should never say the word Megabytes! To all the gamers out there: Get at least 1Gb. Get 2Gb for the newest games like Battlefield 2. Get 3Gb to be safe! For the video editors out there, you know how long it takes. A 1 hour movie at a good compression and quality can be 10 hours work. Get 4Gb of DDR2 and you'll be happy. Also gamers and multimedia workers should look into dual core systems such as Athlon FX2 or Pentium D and Pentium EE. You will not regret it!
    Now that you can do everything you ever wanted to at a blazing fast speed, you want space to store your stuff. Basic users you can get away with lower capacities, but think about this... It will fill up. You will have to either delete something, or upgrade. A $20 to $50 difference in price can quadruple your space. 40Gb is the lowest I would ever go today. 80Gb is normal. 120Gb to 200Gb for those who need lots of music and games. Video people, get at minimum a 300Gb drive. Anything over 500Gb is in a Raid setup, usually Raid-0. That means a 600Gb harddrive actually has two 300Gb drives, but the computer groups them together so you would never know. The advantages are that it is faster, but on the downside if one drive fails, both are useless. There are 2 interfaces a drive uses, ATA or IDE and SATA. SATA are much faster up to 150Mb of data a second (mb/s) while an ATA drive gets around 33 Mb/s. Usually the price is pretty close ($10 difference). It doesn't really matter unless you fall into the gaming rig or multimedia guru category. Then you want SATA for sure. If you are in that bracket, then you have 1 more thing to consider. You might find weird drives like 37.4Gb drives in an expensive computer. That is because normal desktop computer drives spin at 7200rpm (rotations per minute), while these can spin at 10000rpm or even 15000rpm. Obviously faster is better!
    Lastly onboard stuff. Celeron people, you don't need this but it would be better to get a computer with an independant video card. Sound Cards it doesn't matter unless you are an audiophile, because human ears usually aren;t that great compared to what a computer can do. They've been perfecting sound cards for 20 years, and I think they got them right. Now for video cards... There are 2 main flavours: ATI and nVidia. In general, the higher the number, the better. ATI Radeon All in Wonder Pro cards have built in TV tuner so you can watch TV on your computer. Basically, the best right now would be the X850 from ATI and the 7800GTX from nVidia. Those cards are basically matched equally. Then the middle range comparability is Radeon 9800 with nVidia 6200. You want more memory with this too. 128Mb is getting to be low now. 256Mb is the best choice all over. You can get 512Mb and even 1Gb cards but you pay out the wazoo for these, and frankly you won't see any benefit for another 2 or 3 years and by then the price will have dropped enough to get an upgrade.
    That is pretty much it. Support your local eBay computer builders by buying whitebox, and hopefully this guide will inspire some of you to finally get that new system of your dreams. On eBay it is much more affordable! And remember, if it sounds too good to be true, probably is. No $300 BuyIt Now for a P4 EE with 2Gb of RAM and a nVidia 7800GTX!!! Be safe.