I wonder if they got that from the Two Ronnies sketch?
I am surprised how relatively easy it seems to be to find a lot of out of print books for sale on the internet these days.
Though on the few occasions when I indulged the nostalgia for some book I remembered from my youth, I usually realised why they were out of print… 
The Suck Fairy strikes!
A reasonable explanation.
At one time I had seller accounts on Amazon and eBay and sols some old computer games, DVDs and books. I had intended to do a serious culling of my library and sell off anything that I was no longer interested in. If I had gone through with this plan, there would have been a supply of old cold war thrillers and other books which I’m sure are now out of print.
One could sum up this thread thus: every format has its pluses and minuses, with reasons therefor in each format both practical and personal.
I still have seller accounts on eBay and amazon, but I only sell books that I price at $75 or more. Less than that, I just donate them via VVA. It’s not worth the effort otherwise.