I finally decided to play around with Minecraft's trading system, and wanted to get some diamond tools. So I made a bunch of written books, and went to find a Librarian to trade with. He wouldn't take them. The only offer he had was for regular books (or maybe paper, I forget). Back to the crafting table. Eventually, he changed up his offers to the point where he would trade me written books for emeralds – I had my first emerald!
That went on half a dozen or so time, until apparently he'd had his fill of books, and no new offer types appeared.
Which got me to wondering, is trading worth it? For non-enchanted items like diamond tools, would it be faster just to mine for diamonds, than trying to get all the little things needed to trade, hoping you can trade them, and the person doesn't get tired of them…
Best Answer
Trading is random. Period. If one wishes to trade with any kind of reliability (and I use this word extremely loosely) they need to expand and defend a town to mammoth proportions and trade with everyone.
When you perform the only trade a villager offers it expands that villager's trade options, so with enough villagers, all traded, you get many options. If a villager offers multiple options, you must trade his newest option to expand his offers further. Thanks @Kevin Reid for this tip. You don't always get the trade you want, but this greatly increases the chances of doing so.
I should note that sometimes villagers CHANGE their options, which means a trade you really want may not always be available, even if it previously was. The good news, is that villagers only change options or add options when you make trades. So if you have a trade that is too valuable to loose, it would be best to save as much currency (whatever the villager wants) as possible and make a single or few large transactions rather than many small ones.
TL:DR: Trading is not reliable therefore it is not always cost effective.