Your at the tail end of this market... I feel as though I feel like the old TV repair man these days as for computers are so cheap any more that most people buy new.
Reselling used is a tricky business. Can you make money .. yes ... but most people want new and the Chrome books and cheap laptops are making the used laptop market really weak unless its rated for gaming etc.
Money I make these days is not so much in computer hardware repairs anymore as much as it is in Data Recovery Services. If you are able to get peoples data back, that is where the money is.
Other area where money is made these days is with software development and writing software to join one application with another. These days its mostly applications that write to a database in which you bridge the two applications together to a single database, whereas both initially ran stand alone etc.
Money can also be made in getting computers free or cheaply and yanking the mainly CPU and RAM out of them and selling these items. They are light and so shipping isnt bad. DDR3 memory brings good money as well as 4GB DDR2 sticks for those who for example have a motherboard with only 2 memory slots and the mother board will max out on 8GB, they are looking for the 4GB sticks. 2GB sticks also sell pretty well for those who have older boards that max out on 8GB that have 4 slots. Sticks lesser that 2GB for DDR2 dont sell well because people who want to max out their memory and save money are going for maximum memory sticks to achieve the maximum and very rarely is someone looking for 4 x 1GB sticks for 4GB RAM anymore.
CPUs can bring anywhere from $3.00 for a Celeron or Sempron CPU to $55-70 for a Core 2 Quad or Phenom II x6 that is in demand. CPUs for sale for less than $15 I dont bother selling on ebay. Ebay and Amazon are good places to sell used parts with ebay better because you have those who look for auctions about to end and so you might get a bid last minute for a low demand item and sell it for the minimum your listed it for etc.
If your fortunate to get a DOA system with a i3, i5, i7 or modern AMD FX series CPU in it free or dirt cheap and if the motherboard is shot but RAM and CPU are still good, thats easy money right there because these are in demand.
Some power supplies, and hard drives, and video cards bring money as well as specific motherboards. The cases and optical drives are all trash with no market unless you get a nice gaming case that isnt beat up to resell, but shipping is a killer!
Money can also be made in really old collectible computers like an old IBM 5150 etc.
Lastly location, advertising, and gaining peoples trust in your skills to get a name for yourself that is a good one is everything in a service business.
The computer repair business has considerably slowed. You might make some extra money here and there, but there is not enough work in my area to do it full time and survive. Maybe your location will be better, but its going the way of the TV Repair Guy for service involving hardware repairs and the money really now is in people who desperately want their pictures or movies back or business database back that they were lazy and didnt do backups with so they are desperate for their data and you get paid well if your able to save their bottom.
You might make better money running a computer salvage yard if you can get new enough computers cheap to parts out. I find it so much easier selling used parts than a complete used system!
Unfortunately I dont have a good source of dead computers to pick over though and sell CPU and RAM and recycle the rest.
Additionally in some areas its not free to dispose of computer waste and you have to pay by computer a flat fee or by weight to get rid of them. I will gut a system and throw the metal cases and plastic away for free and then bad motherboards I just pay by weight to get rid of and its not that bad or I wait for the annual recycle (earth) day and unload it all for free.
