The easiest way to understanding anything on Yahoo will be through a translation website. Websites like Google Translate and Yahoo BabelFish offer services to translate full pages of text into your native language. You just enter in the link and click on the translate button and it will convert the entire listing for you. You won't need to understand any Japanese at all in order to make a bid. The Japanese site is located at http://auctions.yahoo.co.jp/jp/. You can go there or enter that address into a translation website to play around.
Below is a video walkthrough to a Yahoo listing. It's probably the best way to become familiar with using the site.
This is the more difficult part of understanding Yahoo because it's written by the Seller, and sometimes translation websites don't do a very good job of translating it in a way that you'll understand. This is what will take a little time to learn. After you've translated enough descriptions you'll start to see patterns in the translation that you'll recognize. For example if an item is not included, like a remote control or instructions, the translation will often say "remote defect item." If you're looking at a picture of a game console and you don't see instructions and the translation says "instructions defect item" then you'll start to learn that that means they're not included.
Some translations will be easy such as "No problem in operation" which means the item has been tested. One auction I just translated said, "Concerning operation, several this it executed the game and application, it verified that it is not especially problem." It's a bit odd but that means the computer is in good working order. Another one said, "Unused, operational lack of confirmation." That means it's a new item and it hasn't been tested. After a while you'll leave to understand the cryptic translation after seeing the same sentences so many times in the translation website.
You won't have to understand anything about the payment section of the listing. I can do any kind of payment method the Seller uses, so you can ignore any information about payment.
Some Sellers give a lot of information about shipping costs, some give none at all. If you're using my proxy service you can find out more about shipping costs to me in this article. A lot of times Sellers use COD shipping which means you pay the shipping cost when you receive the item. It's very convenient for the Seller since they don't have to do any research. Some listings will tell you it's COD shipping and they'll give you the box size and their location. From there you can go to the Japan Post website, look up the size on a chart, and see how much shipping will be. Other sellers include a shipping chart showing shipping to all parts of Japan. The video above should give you a good idea of how shipping looks in a Yahoo listing.
"Junk" is a popular phrase on Yahoo Japan. You'll see it often. It can mean a lot of things.... Sometimes they use it as a way to say it's been tested and it doesn't work so it's a piece of junk. Other times they say they've tested it and it works but it's a junk item, meaning it's older and they won't guarantee anything. And finally there are times where they don't mention any testing, they just say "junk item" and you have no idea if it really works or not. If they say it's been tested or if it's usually a realiable item to buy then I usually buy it, but in the end it's a judgement call. You'll see the word "junk" so often that you'll become less afraid of it over time.
No Claim No Return
I talk about that more in this article. A lot of Sellers throw that it at the end of the auction saying you can't return it. But again it's a judgment call. If they say it works and it doesn't then I try to return the item. Sometimes they accept, sometimes they don't. It depends on the Seller. Read the other article for more information about this.