Japanese used car shop or Japanese used auction?

Sometimes, importers ask me which is the cheaper way to buy used cars, from Japanese used car shops or Japanese used car auctions.
This is a little bit of a difficult question.

When the buyers are just the public, of course it’s cheaper to buy from auctions.
They should ask auction agents to bring them to auction houses, and check the cars, ask to bid,then check the successful bidding price.
This is the best way,
but if the buyers are car shop guys like me, sometimes it’s cheaper to buy cars from used car shops than used car auctions.
The reason is the following:

If a seller wants to reduce his inventory, he usually goes to an auction and sells the cars.
Then the buyer pays the successful bidding price to the seller.
Both seller and buyer have to pay the auction fee.
If the seller can find the buyer directly, they can save the auction fee.
Recently, the car shop guys share the information about their inventories on SNS (Social Networking Service) like mixi,
but this isn't the usual way yet as there is not so much information.
Probably it's a waste of time to find a favorable business between buyers and sellers.
It's better to go to used car auctions for car shops too to save time.