Hold you anxiety before leaping on the eBay or any market to buy a Gaming or Desktop PC.
What is the need you want to buy a PC...? Keep the issue right in front on the desk to see the specs you intend to get. At present, every one is interested in compact, high end, multi tasking gadgets where its always a compromise whether to get the portability with fewer dedicated functions, or latest techie with best performance, or cheapest and affordable gadgets to work for our needs. For that to answer instead of me YOU will be the best person to judge.
For someone who think like me, who want a performance like a gaming PC but use it for every other purpose, search eBay NOT with generalised terms like Gaming PC but check for the processor, check for the GPU, check for the MoBo (Motherboard), check for the bundled features and to the finest details you like to have in your 'about-to-buy' PC.
In my case, what I have insisted to buy is a PC with best graphics cards like a NVidia GeForce, which are the best in the market at the moment where I cannot afford for anything like SGI workstations for best graphics I need for my scientific research. I did my market research and made my gaming PC with my intended specs. For that I searched eBay for graphics cards or GPUs, customising the price in descending order and found the features of the expensive and went down the lane of the listing and saw the cheapest and the best fits for my need in the listing. After then, I found the specs of the chosen GPU from its manufacturers website and compatibility and various details and saw the market trend from some of the comparison websites like ciao. Doing that would help you find the market trend and resale value of the commodity that you can sell it later when you don't intend to keep.
This does not to be taken in consideration that your search must start from GPUs but my need suggests me to find specs, which can hold and work in robust graphics for pharmaceutical research like 3D raster. You might start from any end.
Further, try find 'bundles', which are like listings sold as compatible processor and MoBo's together or may be with many more. That would certainly chop over 2/5 to 3/5 of the cost what some of the 'computer-builders' quote a price as (who may be cheapest). A choice of computer case, system cooling, data cables, peripherals, PSU, screws and other bits and bobs would make your ingredients for MAKING YOUR PC.
To be sure what you are doing, its good you check few websites googling with strings like "build your PC", "Make your Computer", "first timer guide to build from scratch", etc.
And, last but not least (well, if you are not versed with) find out a bit more about knowledge base for cracking on to YOUR SILVER DREAM like RAID, SATA, IDE/PATA, etc so that when you build the working software environment you don't panic amidst the way.
GOOD LUCK in making your DREAM PC.