You know the old saying: "If you have to ask...."
It could be any/all of those things. If it was a grey screen with a flashing question mark, your chances would be very good that it was just something a hard drive and a OS reinstall would fix.
But seeing that you (or the seller) didn't mention this, it could be something more. Then again, it could be nothing. That's the problem with buying these things sight unseen - you are rolling the dice.
If you can get the seller to run Apple Diagnostics (hold down D while powering on) and select "Extended Tests" It will run through a battery of tests and then provide you with a diagnostic report.
The downside to this is that you've just shown all the cards to the seller which might now change the price.
Now, while I can't give you a definitive technical answer, I can share with you some of my personal experience:
My daughter has a 13" 2014 MacBook Air which she spilled milk on (trust me, I cried). The logic board from Apple was costing $750+. On eBay, used logic boards were going for $500+. I thought I would try and get a "broken" MBA and see if I could salvage the parts.
Every single one that I purchased (a total of 5 from 5 different sellers) all had more wrong with them than was stated, but all insisted that "they didn't know what was wrong with it." Several had more damage than they let on, and most importantly all had liquid damage but none listed this fact. Liquid damage is very identifiable, especially when they remove the moisture indicator strips. Thank goodness for eBay's and Paypal's buyer protection.
Would I buy non-functioning Macs in the hopes of servicing them? Sure. But I would get my hands on them first.