Ask him if he'd be willing to take it to a mechanic for an appraisal. Should cost under $100. Tell him that you'll be happy to pay whatever the appraisal costs, so long as it comes up with no major issues.

If he's willing to do it then it's probably a good truck and I'd say offer him $4.5k. If he isn't willing to do that then he's lying about it's condition and you should pass.

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2009 Sport 4x4 4 door supercab, 4.0L V6 SOHC, automatic transmission, #1 torsion bar, C code leafs, 96 axle (8.8" 3.73 ratio open diff), towing package, recovery hooks, remote start power package, two tone cloth interior, rear slider window. 31x10.5x15 Goodyear Wrangler Duratrac on American Racing AR767 15x8 rims 4" backspace -12mm offset.