While the server has worked fine until now, that doesn't mean it couldn't improve. The existing player market is very inconvenient, and thus decreases the value of money due to the simple barrier of actually having to find out who has what and how much they are selling for etc etc. Player markets would fix this, as you could just go to the market and buy what people are selling without talking to anyone or requiring anyone but yourself to be online. Everyone would trade more, which I believe would be more fun. Player markets would also add the ability to buy items that are not being sold in the shop, and the price would never be way too low due to the fact that someone somewhere had to obtain it. I feel this point particularly important, as the current shop is very limited in scope. Additionally I do not believe it would ruin the vanilla feel, as players can choose not to participate in the player market. In the very least, it wouldn't ruin the vanilla feel any more than the current system does.
I have no idea what the consequences of adding more plugins in general are though, so I have no argument against this.

About the mending books I am willing to buy when you are online.

Clura summarized my idea very well, thanks.