Any set of wheels and tires with tread is sellable.
Straight bumpers and grillework
Seats that aren't shredded
Leaf springs
Rear axle
Driveshafts
If you ebay, a bunch of electronics are sellable that there's no local market for.
IMO the average beat down, rusty truck has $300-600 worth of stuff depending on a whole lot of things. Half is the condition of the truck, the other is your willingness to tear down and list items before there's a buyer in-hand.
A good example is rear leaf springs. You will about have people fighting over them at $50-75 a pair, loose from the truck. But may not be able to sell them at that price, on the truck. Because pulling them with hand tools is a beeotch.
Successful strategies to generate traffic including stating you'll beat the local u-pull-it in your overall parting out ad. As well as removing parts you think have a market, then doing a good ad with photos, part numbers, or measurements.
And you'll still probably throw away one thing you thought was sellable, long after the truck's gone
good luck with the project, with a little elbow grease there might be a free powertrain (or maybe a pickup box trailer) in your future!