Computers are about the fastest depreciating product most people buy. Especially high-end components like expensive video cards. They'll depreciate slightly faster than a car. You can make money building systems to order for people, but you're highly unlikely to make money building one, using it for a few months, then selling it.
Saving money when you're young is always a good idea. At a 6% interest rate
■saving $10,000 per year from ages 18-26 will result in $1.12 million at retirement (65).
■saving $10,000 per year from ages 20-30 will result in $1.15 million at retirement.
■saving $10,000 per year from ages 30-65 will result in $1.11 million at retirement.
That's right, saving for just 9-11 years when you're young will result in more money at retirement than saving for 35 years when you're older. Yeah I know 6% interest is unrealistic lately, but the long-term average return for investing in an index stock fund is about 7%.
If you do wish to pursue this and are relatively knowledgeable and competent with computers, I'd suggest building/buying systems to order for small businesses. A lot of small business owners don't know enough about computers to decide what type of computer they need, yet they're not big enough to hire a full-time IT person. So you can listen to what they need, and buy an appropriate computer for them and set it up (install updates, Malwarebytes and basic programs they'll need like Office or LibreOffice). Charge a small premium for your time, like $100 per computer or 10%. You don't have to build the systems yourself, you can just order pre-built systems from Dell or HP (you'll learn pretty quickly that time is your most valuable asset).
If you do a good job, they'll refer you to other businesses looking to buy a computer so you'll get more work by word of mouth. And they'll also start asking you for (paid) help when they run into computer problems like hardware failures and virus infections. Just be aware that the IRS and probably your state expect you to pay taxes on the money you make doing this once you make more than about $6350. (You'll have to file a tax return no matter how much you make.)
For a little extra margin, use cash-back sites like ebates, Open a U.S. Bank account and get their Cash+ credit card for your purchases. It gives you 5% cash back on categories you select, one of which is electronics retailers, up to $100 per quarter ($2000 in spending). Another category is bookstores, which works for Amazon. Once you've saved up some money, you can open a Fidelity account, which gets you free checking and checks, free online bill pay, and reimbursement for ATM withdrawals worldwide. Get their credit card which gives 2% cash back on everything, no limit.