I recently faced this situation selling a used Anthem X 29er for a mate who was in a sticky financial situation and desperately needed the money.
I put it up for him on my
eBay account, but (fortunately as it turns out) I let myself be successfully pestered into selling it off
eBay by a persistent buyer.
I took the bike for a test ride before listing and all seemed fine to me, including the fork lock out. I almost purchased it myself in fact, the only reason I didn't was that the bike was a size smaller than my usual ride and I thought I'd be better test riding the Large just to be sure.
Seller got very cranky with me - part of my test ride involved riding down a few flights of stairs, something any mtb should be able to take in its stride. and which I do for fun / skills sharpening on my own bikes nearly every weekend. Our local trail has rougher descents that we ride every weekend in any case. I suppose that reaction should have been a clue, in retrospect.
It turns out the fork stanchions had a slight (3mm) bend in them, which the buyer only picked up when he took it to a LBS to get serviced a week later due to lockout problems. I certainly didn't notice it. If I had, it would have either been disclosed or more likely returned to my mate, his desperation notwithstanding.
Fortunately I had disclosed the fact I was selling on another's behalf and, with the money out of my hands that night, there wasn't a whole lot I could do for the guy who bought it. My mate had decamped to Perth from Sydney pursuing a job offer and had no spare cash in any case. More than likely he'd have blamed me for doing "something" to the bike.

Quite possibly the buyer, a newb, stacked the bike after he took it away from my place, but the bike shop mechanic's reported comments did provide a more credible explanation of why the front wheel had been re-laced than my mate's "they all came loose and were replaced by KOM2 under warranty"
Unfortunately I'll never know the truth, but as a result I won't be selling on behalf of another again.
On the other hand, I've successfully bought a number bikes and other second hand items on
ebay, but would never buy a carbon bike second hand.