There is no domestic ebay in Japan, there hasn't been since the early 2000's those goods are for sale via ebay.com.. you can find pretty much all of those "expensive" ebay sales listed on domestic JP auction sites for half the price of ebay (broadly speaking it's not the goods which are expensive, it's ebay's costs for JP sellers) as previously mentioned Japan does have plenty of auction sites to choose from.. Yahoo, Mercari ect but Buyee isn't one of them.. Buyee is a proxy service which provides a link to the listings in the same vein as FromJapan, Jauce, ZenMarket ect..
Goods in JP are generally cheaper tho, there is no 20% VAT just 10% consumption tax for domestic sales.. for example, my last purchase, a NOS
Syncros seatpost cost £32
If you want to buy anything just fwd the URL to one of those previously mentioned forwarding services, here is a costing for that^ Syncros
Purchase price ¥5350 (£32) comision ¥300 (£1.95) to ZenMarket (ikr?) and ¥2450 (£16) for EMS shipping for 360g from JP to UK.. so £50 in total, it would cost nearly twice that from ebay.co.uk. If you need to ask any questions this can be done via your proxy service, they're often multilingual.. You only need a proxy service because you can not sign into Yahoo Auctions without a domestic JP shipping address.
There is a culture of collecting, displaying and caring for things in JP you see it in the collectable toy world, sneakers, vintage denim, pottery, musical instruments ect ect Hence it's a perfect place to buy your well cared for bike components, i've been buying from JP since 2005 without a hitch.