I bet you could sell it fairly easily. Trucks like that, especially with dump beds are always in demand.
It would be worth a lot more if it was running and operable. If it has not been run during the 12 years, it might not be too easy to start. The first thing I would do is to remove the spark plugs, to make sure they look OK. While they are out, I would squirt a teaspoon or so of oil in each cylinder. Then I would try to get the engine to turn over, first by pulling on the fan belt, and if that didn't work, by using a wrench on the front pulley nut. Once I was sure that the engine was free, I would put the plugs back in. If the battery has not been kept up, it is probably ruined and would need to be replaced. With a working battery, I would test for spark and would probably need to file the points and readjust them for the proper gap. If there has been gasoline in the tank, it is probably real bad. I would try to siphon as much out as possible, or if there is a drain plug, I would drain the tank that way. Then I would put in about 2 or 3 gallons of new gas. The engine should turn over well with the starter, so I would test the fuel pump by removing the line to the carb and have someone turn the engine over while I collected about a quart of gas. Make sure the coil wire is out of the coil while doing this, to prevent a spark and possible fire. If the fuel delivery is OK, the engine might start, so I would return the fuel line to the carb and put the coil wire back in the coil. With the choke pulled out, I would try to start the engine. It is important that the truck be out of gear, since the clutch disk might have rusted to the clutch or flywheel, and if it started in gear, it might move the truck. If the engine starts, it will smoke like crazy for a couple of minutes, from the oil squirted in the cylinders, so you want to have it well ventilated. This would be the time to check clutch operation. If the transmission will go into gear with the clutch pedal pressed down, great! If not, you will probably need to pull the truck around with a tractor to break it loose. That might also need to be done if it will not start with the starter. But if there is adequate fuel, decent spark and some compression, sooner or later the truck should start. If the cooling system had been drained, don't run it very long without new coolant, since that might damage the engine badly.
The value of your truck depends on condition and your local market (which might be very large because of the internet). If the truck is in decent condition, with a usable bed and operating hydraulic hoist and able to run down the highway and with operating brakes, I would bet that it is worth at least $1500 to $2000. Without it running, I would be surprised to get half that. If the interior is not all mousey, the paint is pretty good and the bed is all metal or is really nice wood, it might go as high as $5000. It really helps that it has been stored inside. Hopefully the mice have not been too hard on it, though.
If you need a dump trailer, that would be one way to build one, assuming you have something else to pull it with. But I doubt that such a trailer would be worth more than the complete truck, if you plan to sell it, and it would be a lot of work to build it into a usable trailer. And it seems a shame to tear up what sounds like a neat, restorable and usable truck. Good luck!