I'm very glad Rick pointed out all the issues with trading on Yahoo.jp. There is a very good reason why it's not done more often.
It seems every now and then, someone will post up how cheap something on Yahoo.jp is giving the perception that yahoo.jp is a great shopping ground for cheap boomboxes. That's only part of the whole story. Keep in mind that there are fees. Lots of fees. Your $10 radio could eventually become a $300 purchase. And if it comes shattered, not working, missing parts, etc. etc..... doesn't matter -- your bought it and it's now yours. How many purchases total will it take to compensate for the bad ones. The risk is not insignificant and not for the faint of heart. Make sure you have the stomache for this type of transaction and can afford to throw money away. For example, jauce is frequenty cited as a proxy. So when you fund your account with play money via paypal to the jauce system, paypal will give you a punitive 2%-3% hit on the conversion rate, just for fun. Then when the funds in jauce are used, you are assessed a bank fee. Then when you win an item, you are charged 800 yen PLUS 8% commission. Then you need to pay for the item. Then there is the cost of the domestic shipping from the seller to jauce. Then there is the repacking fee, which is either 1200 yen or 2400 yen. Then there is the overseas shipping cost to your destination, could run anywhere from $120usd to $260usd depending upon what you bought. Even the smallest boombox could run well over $100 shipping. Of course sea shipping is an option if you want, but then repacking isn't an option and they will charge you an additional handling FEE of 250yen per kg of product weight. An RX-7700 would weigh in at a whopping 15kg packed which adds an additional 3750 yen handling fee in addition to the normal sea shipping charge. In case you are wondering, $1 only buys you about 72-75 yen nowadays but you need to factor in your "after-tax" purchasing power since remember, paypal already punished you with the poorer than normal conversion rate to begin with.
So how much are the fees? Just read through the list above yourself and calculate -- I get a headache just looking at the fees list. Don't believe all those fees? Just go the jauce yourself and read the faq's. It's all in there.