I would like to have player markets. Usually I see this done in the form of a chest with a sign that says a price for an item and you click on the sign, but really any way at all for players to sell items to others both easily and also where they don't have to be online would work.
This Idea has been proposed in the past by a number of people. Personally I don't see it necessary for the server to have this feature added as we've gone for years now without it and things have gone fine without the additional plugin, also the way money and trading is used in-game works perfectly fine as it is. Players trade with each other or have the option to go directly to the /warp shops and I see no problem in how this setup works. I remember deekay stating in the past that he doesn't want the server to stray too far from a vanilla-style server than it already is - Which I totally agree with. (Though that was a long time ago he said that, his opinion may have changed.)
With the mending book situation, players on the server already have villagers with the mending book trade. (I myself have one and would be more than happy to trade mending books with you)