Edit: I didn't read the description... selling works too! Twice as fast! Wish I'd noticed that!
Rather than try to download worlds for this one, you can just generate your own, spawning right next to a village. Use the seed "camero". (I set it to Peaceful, but if you're fast enough, you'll be able to do it in one day.) xCANExINSANEx93 suggested to use "village pls" in another solution, and that works well too.
Turn around when you spawn, and grab two tree trunks. You can probably get away with one, but I took two.
Now go toward the village. In the leftmost house (just past the first one on the right), there are bookshelves and a crafting table. Make a hoe.
Now run around the town harvesting everything. There are around 10 fields. (Don't worry about going outside of town. There isn't enough there to waste your time.
You're going to be looking for only two things: potatoes and wheat. Be careful, because some of the stuff will fall in the water. You're going to need every bit you can get.
Once you've harvested a field, move to the next. Some of the farmers will start resowing the fields. Let them. When you're finished with the rest of the fields, go back to those because the farmers were sowing them with...potatoes. More for you!
Now trade them for emeralds. You should have roughly 37-38 wheat and over 90 potatoes. What's important is the exchange rate. At least one farmer will trade an emerald for 18 wheat, and another will trade potatoes for 15. Don't trade for more.
If you have enough food, you should be able to trade for 8 emeralds. If not, you should get at least 7. Then... turn around and sell them for bread or pumpkin pie. In one run, you should be able to buy/sell a total of 14-16 emeralds.
Now save & quit, and start with a fresh world. Hopefully you'll be able to get this by the end of your second run. Each one takes around 10 minutes, so this should take around a half an hour.