If you're a member of AOPA, sign in to their website, under the tab "Aircraft & Ownership", select "Aircraft Valuation".
I do a quick evaluation of an airplane by figuring how much the remaining time of the engine is worth, then subtract it from the asking price. The result is what they're asking for the hull + avionics. In this case, the remaining engine hours (2400 - 880) multiplied by $7.50 ($18K cost of overhaul / 2400 TBO) is worth $11,400. They're asking $21,600 for the hull & avionics. Too much.