I woulda done it too. Lets him know that it's just unreasonably priced. Now if it was a complete custom truck at a show for sale, I could see it going for that price, but on craigslist, really doubt it.
I do the same thing. I seen someone advertising their base model high mileage Audi A6 2.8 for the same price as a low mileage 4.2 and I responded telling him he's on crack. Just 'cause he threw on some aftermarket pieces he thinks it's worth so much. It wasn't even lowered, didn't have a performance exhaust or anything special. Just a few things here and there. I usually ignore idiots but sometimes I just have to bitch slap them to remind them how stupid they are. Like some of these other roofing companies that look for master shinglers yet are only willing to pay labourer wages. The Internet has allowed idiots to have a voice.