Put very simply, Yahoo Auction is the ebay of Japan. The main difference being Yahoo Auction is in Japanese. If you can't read Japanese, try using Google Translate to translate the pages into English, or a Chrome extension like Rikaikun. They both work pretty well on the Yahoo Auction site.
So you've registered for an account on Yahoo Auction, now what to do?
Just like on ebay, use the category search function to narrow down the auction list to a number of specifically relevant items.
Case Example: Buying a Second Hand Road Bike
Let's assume you are someone new to road bikes, you like the idea of riding with mates through the wilds of Saitama or Kanegawa or Yamanashi (or any other place outside a big city), but don't want to commit too much money to your first bike. If this is you, you are looking for a relatively cheap, probably second hand, complete (as in 'ready-to-ride') road bike, with 'good enough' components, for, say, around 50,000 yen.
If so, you will use the category search on Yahoo Auction something like this:
スポーツ、レジャー > スポーツ別 > 自転車、サイクリング > 車体 > ロードバイク
sports leisure > sport another (i.e., by sport) > bicycle, cycling > body (i.e., complete bikes) > road bike
This will show you a list of bike size categories. If you are unsure about which size is right for you at this point, you will ask a question in The Helpful Thread and someone will tell you. For quick reference, someone 5'9 (177 cm) would be okay riding a 540 mm size bike, so would select the '540 mm ~' category (the tilde '~' mark representing 'and above'). Someone a couple of inches taller, around 180 cm, would select the '560 mm ~' category.
You now select a price range. Set the top end to 50,000 yen, but also set the bottom end to something like 20,000 yen. This will remove a lot of clutter from your search.
Now select the 'second hand' option (中古). This excludes 'brand new' (新品) bikes. At this price point, any brand new bike is unlikely to be of 'good enough' quality for riding through the mountains with your new cycling mates.
Now click on 'Bid' (入札) above the column showing the number of current bids on any item. This will sort your list to show items in descending order of bids placed. i.e., in descending order of what people are showing an interest in buying. Since you are new to bike buying it's a good idea to follow the lead of others.
Now you wait, and watch, and read. You wait a while because this is not an impulse buy. You watch a while using the 'watchlist' (ウォッチリスト) function to see how prices behave on certain bikes. And you read the descriptions of these certain bikes to learn a bit more about them. If there are things you are not sure of, either use Google or ask in The Helpful Thread.
Some points to bear in mind:
• Japan is hilly, so you probably want a 'compact chainset'
• Japan likes registering bicycles, so you want to get a signed statement of sale (receipt) from the seller, along with the Registration documents for the bike if available, that you will then take to a bike shop where Registration of the bike will be transferred to you for 500 yen.
• The crap bikes on Yahoo Auction will probably attract few or no bids
• There will be delivery charges, so factor in how much delivery will be into the price
• Take it easy and wait a while - the right bike is unlikely to come along right away. Bid from a place of calm tranquility.
• Most auctions end on a Sunday evening
• Before you decide to bid on something for reals, why not ask The Helpful Thread to see what the forum thinks!