I have tried to sell them in the past, but people are morons and want to give me 10k for a high tier book. WTF, I can make 10k in about 20 min on a legend thief.
I double that (except for the morons part). My mage isn't even legend, but my typical cash run is 7-10k and it takes me like 10 minutes to visit all my favourite npc creditors. What is even better, I can do it between the zones when the group moves and reforms or even during the zone (relo away, hit the cash spot, relo back, all within 1 tick while the healers mem). Certainly, then I have to wait 1-2 hours for the repop - but who cares. One hour for a zone, 10 minutes for the cash run; repeat.
As the result, there is absolutely no point for me to sell a book (even the lowest one) for under ~1-2k. I will either give it away for free or drop it in the nearest guild or let it rot.
On the other hand, the higher tier books are much more valuable. I'm not that desperate to get 10k coins, I would rather wait for a clannie who recently made a mage as their 5-th char to log on a study it. If they don't and the book rots away - I will be certainly sorry for the clannie not getting the spell, but not at all for the money I missed.
To conclude, the players who can load a certain quality book can also load a certain amount of cash fairly fast. Unless selling the book to the world merchant provides a suficient profit, they wouldn't bother selling it.
So would you be happy if books were bought by the world merchant for 2 coins and sold for several thousands?
Is this truly about cycling more equipment or is it about people wanting profit from things they have popped?
There is no "would" here. If the world merchant buys the books for 1 coin, the sell price is unimportant because it never actually gets a book. Similarly, this IS about cycling more equipment, but unless those who can provide that equipment have some motivation to do it, the system will not work.
Thus, imho the world merchant should buy the books at no less than:
lowest tier : 1-3k
mid tier : 5-10k
high tier : 20-25k
elite tier (nightmare) : 50-100k