Buying a "certified" used car

[pvn]

Dear OOT Geniuses:

I want to buy a used Honda Odyssey. I have been watching the dealers around here for a while, and one nearby has one I like (2004 EX-L, 55K miles, loaded with everything except sunroof and DVD player (both of which I do NOT want anyway)).

I have financing already arranged through my credit union. When I negotiate the price, I tell them and they fedex a check.

My questions are mostly about the negotiation process.

The dealer is asking $20,900 for the van, which is below the "blue book" value of around $22-$24k but above the actual selling prices I'm observing on ebay (comprably equipped vans with significantly fewer miles are going for around $20k). My credit union, which is very helpful, told me that the "loan value" of the van with the options and milage is $16,900 - this is supposedly the amount the dealer would pay if he bought it at wholesale auction, and this is where the credit union suggested I open negotiations.

After that, though, it gets fuzzy. Of course they're going to laugh at the first offer, which is fine, but then they're going to come back with something like $20k, say. What's my next move? The credit union's buying guide literally suggested upping my offer by twenty five dollars. In their example, the dealer's asking price was $22,000, the opening offer was $17,000, the dealer's counter offer was $21,000, and the buyer's next offer was "how about $17,025? Well, $17,030?" The idea being to show that you're a "tough negotiator" I suppose. Does this work? I don't mind walking out on this van if they won't come down on the price, as another one will show up in a week or two, but I don't want to be so unreasonable that they won't deal with me in the future.

What price can I reasonably expect to get out of them, and what is the most effective way to do it?