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I find the best way, and it produces a lot more results generally, is to use google translate to convert what you are after into Japanese then paste that directly into yahoo.co.jp .. see what you find (don't forget to use always translate option in your web browser or you have to right click and select translate to English a lot) when you find what you want copy the Auction ID and paste it into the search function of your selected proxy service, Buyee, Fromjapan, Japmart etc., and off you go.

Prices are not always a bargain --- have local reference site to judge against and consider the extra costs you will incur when adding up the final delivered to you price.
Never buy something that has engine in the description.. you will most likely lose it as the Japanese are funny about engine models and shipping.
If you are buying a full kit NIB i would use FromJapan as they do not remove items like grease . small shock oil bottles etc. which other do so .. not NIB anymore.. very annoying.
Be very careful about what is in the box as they are prone to urrrm Stretching the Truth a lot
Fees, Charges, Shipping and UK taxes can easily double you winning bid price.
They have a strange Auto Extension function which means sought after items auctions can go on for a long time and they are prone to bid in the last 3-4 mins.. so keep an eye out.
Only bid once at a time to your max(well max at that time) as sometimes they use different ID's and you can find yourself bidding against yourself.
You won't make a business out of this but you will find stuff that is not readily available elsewhere in the world generally and you will get some serious bargains sometimes.

There are more sites out there in Japan you just have to dig to find a proxy that will allow bidding on them.

Hope that helps but don't go crazy as you will have other fees and shipping costs to pay after you win an auction which can easily double the price or more if you don't keep an eye on it.

Good luck.
Al
UPDATE :Beware of New In Bag items I just received two New In Bag body sets , one was missing small parts that are near impossible to find and the other had the rare decals swapped out for very common ones re-re ones which were turned upside down so I could not see them from the pictures posted with the auction but they both stated NIB and I paid accordingly for that alone. Sucks to be honest.

Be very careful.. crooks and dodgy chancers abound across the net.. heck years ago someone came up with the scam of selling "Fake Rice" which was made from recycled plastic... the lengths some people will go to to make a buck is crazy. 

I will backtrack and find the sellers and will post their details here.   
Al

UPDATE: Grades of goods in Japanese Auctions.

These are common grading numbers/letters used across nearly all Japanese auctions not just real cars so they do apply to models, clothes etc. they are normally in the description.How do you read car auction grades?Understanding the Auction Grade

1. Grade 6 or S – In most cases, cars with this grade are brand new.
2. Grade 5: Good as new.
3. Grade 4.5: Slightly used condition.
4. Grade 4: Used condition with minimal blemishes.
5. Grade 3.5: Good condition with noticeable imperfections.
6. Grade 3: Possible light damage with average condition marks.

Update: Yahoo Japan Has banned UK so you will need to use a vpn client set to use either Japan or US to be able to view yahoo japan auction pages