I don't know how it works in other countries, but in most parts of the USA you can register with your city to get a vendor's license. This is usually a small fee ($100 or less), and with that license you can order PC hardware for resale from distributors. This has two main advantages:
1. If you buy the hardware for resale, you do not have to pay sales tax on it.
2. If you buy the hardware from a distributor you can get a lower price on the components than if you bought them at retail (even from the cheaper retail outlets).
3. You may be able to take advantage of specials offerred to resellers only through distributors like buy one get one free, deep discounts for volume purchases, combo deals, etc.
Now the downside of being a PC reseller:
1. You are competing against companies like Dell, HP, Gateway, etc. They can offer a lot more functionality or performance at the same price point as you because they can buy components in bulk and have very efficient manufacturing/assembly processes.
2. Unless you have a special hook, niche market, or feature, then you will largely be competing with others based on price until you develop a good name for yourself. And since you're in this to make money, trying to compete with the big boys on price is going to make it hard to remain profitable. Which leads to number 3.
3. Profit margins in the PC industry are razor thin and support costs are high, even for the big companies that buy in bulk. The gross profit margin on a complete system is usually 10% or less. If you can sell extended warranties or more specialized (and profitable) hardware with the system, then that gets higher. But if you make $50 off of a $500 sale, then figure in your time and expenses you may decide that it isn't worth the effort. Especially if you have to spend any time at all supporting it the product post-sale.
In the example above let's assume that it takes you 1 hour to assemble, install, and configure the $500 PC (that's optimistic). Let's also say it costs you $450 in components. Your gross profit is $50. Let's say that you spend another $10 in overhead (it costs money to ship the components to you, post the auction or pay for a web host or whatever. All of that effort has netted you $40 for an hours work. Not bad.
But then the user receives the PC and has a problem, and you spend and hour on the phone with them fixing the issue. Now your pay has dropped to about $20 an hour. What if there is a problem where the mainboard dies and needs to be replaced? The customer ships it back to you (at their cost), but you have to get a replacement or repaired mainboard from the manufacturer, for which you will likely pay for shipping that mainboard back to the manufacturer, then shipping the repaired PC back to the customer. And by this point you will have probably spent at least 3 hours working on the PC, and your initial $40 profit has dwindled to $10-$15 (if you're lucky). Now you're making less than minimum wage.
And of course this is all multiplied if you manage to stay in business long enough to start dealing in volume. Because instead of selling 25 PCs a week (which you could probably do on your own), you are selling 50 PCs a week and need to hire someone to help you build, configure, and ship them. And while you might be able to get by if you don't take a paycheck during a slow week (or a particularly costly week), you can't get away with not paying employees. And then there's extra costs associataed with having employees, even if you don't offer benefits (mainly employer's taxes).
Based on past experience, I will say that it is very difficult to do a PC business this way. There are some people who can be successful at it, but I think that most people who try it for awhile end up giving it up within a couple of years. It used to be a lot easier 7-10 years ago before the boom in computer sales (back when Dell and Gateway were small operations just making a name for themselves). Computer magazines were filled with literally hundreds of companies selling systems, but as the market became more commoditized the average price of a new PC dropped from $2500 to $600, and most of those companies went under or were bought up by the bigger players that are still here today.