You do NOT want to buy OEM-based laptops used, and without warranty or knowing how much and how hard to find the parts might be.
OEM's usually have proprietary deals with MFG's and charge a SERIOUS bundle/premium for parts. Even older parts, if you can find them for sale, will negate the savings on the price to buy the machine used and from a friend.
I've got a few Dell's and HP's and probably the best solution, is to sell them back to the firms when it's time to upgrade. If my mobo burned out or GPU, I'm 2-8 months from possibly finding a new one, even though I've next day replacement service for another 2 years. So that leaves me having to research and track down warehouses in 2nd world countries, with translators, to find the possible parts I need.
In East Europe, it took me 2 weeks to find a replacement GPU when mine cooked itself because Dell wasn't going to send me a replacement fan when the old one started failing. My warranty was via USA, where NO PARTS were available, but I got lucky in that over in another country a persisiten local E.Europe rep was able to find me a refurb GPU like mine, that was sitting in a box on a shelf almost forgotten at some little computer firm.
I got lucky, real lucky.
This is WHY Dell simply replaces their machines with a whole new one most of the time- they only make so many extra parts for one's that fail, to keep costs down with the MFG's. When they run out of parts, it's cheaper to send a different laptop.
When you buy one that is no longer on warranty, you're really out on your own- as getting parts, unless you have a 2nd machine, or get real lucky on Ebay, are really a pain to find...unless you find something like someone who had a different part fail and is in the same boat as you. Piecing together a few failed machines is always something fun!