First of all, make sure the seller has a clear title in hand. If not, don’t bother going any further. If he says he does, then ask to see the title first thing when you go to see the van. Compare the VIN number on it to the one on the van. Again, if he can’t show it or it doesn’t match the vehicle, walk away.
Ask ahead of time if you’ll be able to take the van for a test drive? If not, move on. Go when there's plenty of daylight. Never buy a vehicle in the dark—literally and figuratively.
I think you can tell a lot about the condition of a vehicle from the condition of the owner and his surroundings. People who take care of themselves and the things in their lives tend to take care of their vehicles. There are exceptions, like the guy who is so focused on cars and trucks and all things mechanical that everything else is unimportant.
Look at the ground under the van. Are there oily spots or puddles of something other than rain?
Open the door and take a good whiff. What do you smell? Essentially nothing? Maybe the lingering aroma of cleaning products? (Not pleasant, but generally a good sign.) Or does it smell musty, mildewed, moldy? Like food gone bad? Like car-sick kids or incontinent animals/humans? Like something you never hope to smell again? Does it smell like the engine compartment? Like exhaust? It shouldn’t.
Did all the doors open easily and swing without binding or creaking? Look at the weatherstripping around the doors and windows. Is it in place or hanging loose in spots? Is it flexible or dry and brittle? Do the windows roll up and down easily? Do the doors lock and unlock easily—with the buttons and with the key? Are there at least two keys?
Look at the upholstery and carpeting. Does it look clean or like a disease farm? Is the wear and tear consistent with the vehicle mileage?
Speaking of mileage, beware if it’s an older vehicle with the type of odometer that has numbers on little wheels. They can be turned back. Also, if there are only five digits other than the tenths, you have no way of knowing (other than the seller’s word) whether it’s 73,294 miles, 173,294 miles, 273,294 miles or more.
Is the driver seat sufficiently comfortable? Do the adjustments work? Do all the controls and buttons work? Lights, signals (get out and check), climate control, radio, power mirrors, power windows and locks (if so equipped)? Is anything missing, like knobs, buttons, levers, cranks, mirrors? Turn the steering wheel. Is there a lot of slack?
If it starts, does it idle easily? Does it rev smoothly? Or are there weird noises like clunking, grinding, clicking and whining? Any weird smells, like exhaust, hot antifreeze, hot wiring, or dead rodents in the air ducts? Does the climate control actually provide heat and cold like it should?
If you’ve never driven something the size of a van, it will feel different. It won’t turn as sharply or stop as quickly. You’ll have more blind spots. It will feel top heavy. It will feel huge. But you’ll eventually get used to it.
Check the gauges. Is the temperature in the lower half of the scale? Is the alternator putting out about 14 volts? Is the oil pressure around 40 PSI? Is the Check Engine light off?
Do all the electrical items still work, or do they flicker on and off when you go over bumps or change speed? Are there still no odd smells?
Okay, so maybe everything seems healthy about the van. Good, but still have a pro check it out before making any offers. A mechanic’s informed opinion can give you a better idea of the van’s actual value. And he can tell you what things might become a problem in the future.
Or maybe there are some problems. You don’t need to know the sources of the problems or what it would take to correct them. You just need to know they exist. If the seller says they’re all simple and cheap to fix, ask why he hadn’t taken care of them, then. Knowing there are problems, you need to decide whether to spend the money to have a mechanic check it out and tell you whether the repair costs are worth it, or whether to save the time and money and just walk away and keep looking.