I don't buy YA anymore because I expect it to be mediocre. On three separate occasions I was told a Taylor Jenkins Reid book was amazing so I went out and bought it... I won't be fooled a fourth time Taylor! Now I only get YA on Kindle Unlimited or from the library app. Same goes for the new Marian Keyes, Mhairi MacFarlane, etc. Always pleasant content but you know what you're getting. I don't need to own them. And thrillers which I won't reread because I already know whodunnit.
But... I will buy brand new hardbacks simply because they have beautiful cover designs and I'm concerned that if I wait, the paperback will come out and have an ugly cover, which has sometimes happened (example - The Europeans by Orlando Figes www.amazon.co.uk/Europeans-Three-Making-Cosmopolitan-Culture/dp/0141979437)?tag=mumsnetforu03-21, or just diminished in a smaller size (example - The Start of Something by Stuart Dybek www.amazon.co.uk/Start-Something-Selected-Stories-Stuart/dp/1784702854/)?tag=mumsnetforu03-21
I have also been known to pay too much to get a book with a specific cover that is from another country and out of print. And I have bought beautiful limited editions that I couldn't stand to mess up by reading them (LOTR) so I went out and bought charity shop paperbacks for the actual reading part 
I'm honestly also quite content with my system though. No disposable paperbacks piling up around me, and I like looking at my pretty book collection.
Anyone else have arbitrary book buying rules?