Are you supposed to bargain when buying used rental car?

I'm trying to expedite a lot of things, in a pretty narrow window, and decided to buy a used rental car. I have no idea if you're supposed to bargain them down or just leave it. The price looked good per Kelly Blue Book but then I realized I had forgotten taxes and other fees. So I'm thinkin, "OK, 11K," but it probably will be more like $13K (after paying my 2-week rental after totaling a car).

I'm pretty OK with just costing every thing out and bringing back a cashier's check, but would I be dumb to not bargain down a decent KBB value?

I know it can't hurt to offer less, but I hate this aspect of car-buying.

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