Auctions, find some in your area. They'll likely have surplus from schools, government, and businesses. Pallet of ~20 computers should go for 300 or so. Likely older ~3 year old systems that have been returned after lease. Single system for $90? Probably not going to happen, you got to buy in bulk and the used market is mostly auction.
It's what I do for a living, my business buys and sells used electronics. You can make money but you'd better know what you're doing. Meaning you can look at a pallet stacked with computers and even though you can only see the top 4 systems on the pallet and the sides and corners of the rest, you can tell me the exact model number and specifications of what's inside. If you know your **it there's no possible way for us to mess with you but if you can't do that the regulars, like me, will make sure you're out of business real quick by making you pay too much for the stuff. On the off chance that you catch on and we do win the bid? We'll just signal the auctioneer to re-auction that lot and they'll do it because we pay the auction house a whole lot more money than you do.
Make sure it's what you want to do, it's profitable, it can be fun, but there's far easier ways to make a buck. You'll be working Saturday's as most auctions are on the weekend. Most auctions are also held in the middle of nowhere so being outside in the summer when it's 108 degrees? That's a normal day at work. The stuff you buy is generally bought as you see it, where you see it, so you'll be man handling the stuff you purchase into your vehicle.