Van Life Trader is the place to go, unless you plan to build out yourself (in which case the usual used vehicle markets are a better/cheaper bet - ebay, craigslist, etc). Andy's warning is a good one, but no need to be too scared... From a safety perspective, I think there are a few things to look out for, but all are pretty straightforward.
- Electrical - Risk is you burn down the van, like Andy said. What you want to check is that the wiring is done well and properly fused and gauged. You don't need to check every wire to see what sort of craftsmanship went into it. Look for large, high-current wires and run the basic calculations to see if they're properly sized and fused. Just reading a few build guides on vanlife electrical systems should give you enough info to assess whether it's sketchy or not. This is slightly more critical if the van has high-current devices like an induction stovetop, in which case you should check the wiring all the way to those devices.
- Heating - Risk is you fill the van with CO while sleeping. If you have any sort of heater, check that the combustion loop is properly vented. Install a CO detector if there isn't one already. Better yet, have two.
- Propane - Risk is you blow yourself up. If you have a propane system, check for leaks and that there's a proper, normally-closed solenoid installed. Install a propane detector if you don't already have one.
- Seating - Risk is that you get in an accident and discover your seat wasn't properly bolted to the frame. If you only have a driver and passenger's seat, probably not a big issue, but check any seats that were added after the fact if you're going to use them while driving.
- Furniture - Risk is that it comes undone in a crash. Just check the attachment approach and give the cabinets a good shake to get a sense of how solid things are.
- GVW - If the build is complex and has lots of added furniture and components, it's probably a good idea to go to a weigh station and make sure it doesn't exceed the safe load.
Other than those risks, I don't think there's anything too critical in the build that could cause death or bodily injury. Good luck finding a van!