The seller is definitely starting high and expecting to haggle. Your offer of 1800 is not way out of line. Or maybe seller really believes that’s what it’s worth. In that case, you should keep looking—unless you just really love this particular bike for some reason.
Maybe the seller will get upset and say you’re coming with a low ball offer. It doesn’t hurt to try though. Both buying and selling through local online ads, I’ve almost always gone through a negotiation process, two or three offers and counter offers. Don’t feel bad; that’s just how it’s done.