Yes, Shill bidding is rampant on Yahoo! Japan and is a huge problem.
If you win it and its at your max bid, then most likely the seller is using other accounts to bid up the auction and then canceling the bids from their selling account. Nothing you can really do about it.
The best way to protect yourself is to know the true value of the item you are bidding on, how much it regularly sells for, and don't ever bid a single yen more.
If it's for a specific item, it's ok to lowball bid and then come back later and see how much it actually sold for so you have an idea of how much you should be bidding.
This is what I frequently do. Lowball bid everything I want, look and see what it went for, and next time bid a bit more.
But on some things, especially collections/"junk"/bulk sale items it gets very difficult to estimate what the price should be.
For example, I see idol DVDs selling for ~500 yen so the collection/bulk sale of those I might estimate at 500 yen x the number of DVDs and then divide it by 2 and then still have the bidding price bid all the way up to my max by the seller's shill bidder accounts.
For those items that I low ball bid on, sometimes I win it at a ridiculously low price. 100 Yen for a stack of blurays or DVDs, and the seller's shill bid accounts even worked their hardest to bid it up to that 100 yen. And I get all excited. Right up until the seller cancels the whole auction for some BS reason and I never get the items anyway.
Yahoo! Japan is rigged against the buyer so don't ever think you're going to get a ridiculous great deal.
I don't think I ever have.
You always end up paying around what the market price is for the item, or the seller cancels the item if it doesn't go up to that level. OR in a much worse situation you pay a lot more than market price for the item because you didn't research how much it's actually worth and the seller used shill bids to screw you over.
So really, these are the possibilities:
1. You're the highest bidder and the seller uses shill accounts to bid it up to your max, but doesn't agree to the price and cancels the whole auction.
2. You're the highest bidder and the seller uses shill accounts to bid it up to your max, and agrees to the price so you get the item.
3. You're the highest bidder and the seller doesn't use shill accounts to bid it up to your max, and just cancels the auction because it's not anywhere near what the market pays for the items.
4. You're the highest bidder and the seller didn't need to use shill accounts because it's a popular item that a lot of people are wanting, and bidding on, and so it will just automatically go for the market value and you get the item.
5. You're not the highest bidder and you just don't get the item. And maybe the highest bidder also doesn't get it either because the seller cancelled it (see 1-3 above).
Yahoo! Auctions in a nutshell.
Also, a lot of sites like Zenmarket will ban any sellers that cancel auctions after you win it. So when they relist it after cancelling it, you can't bid on it from that seller again. And nobody else that uses the site can bid on anything that seller sells. Seller doesn't care.
Just FYI, if you low ball bid something and seller cancels, you might not be able to bid on it from the same seller after it's relisted because you got the seller banned from Zenmarket. Which might be bad if it's a very rare item and you already know what it should sell for and decide to low ball bid it anyway trying to get a crazy good deal. Happened to me one time on an item that I waited over a year for to show up for auction and then I couldn't bid on it because seller got banned after cancelling my winning auction with a lowball bid previously on the same item. So somebody else got it for less money than I was actually willing to pay for it AND on top of that I had to wait a year and a half before another one went up for auction.
So extremely rare items that you check every day and only see go up for sale once a year or so, it's not a good idea to be low ball bidding it.
Been using it for years, hope this helps somebody. Maybe I should write a book!
