I always like going to the dealership. They will do a trade-in and help you get into a new van. Prices are really high about now so check prices online to several places and then go from there. I bought from a dealership and got my minivan used but that was a few years ago. I have a great experience and it was pretty quick. I know I paid a bit more had I bought from a private seller but buying from a dealership is a great way to ensure you don't get stuck with a lemon or end up getting mugged or scammed.
A dealership means they will stand behind the sale so if it goes wrong they are still there tomorrow to sue! Posters on craigslist have no paper trail so once the sale is done they are onto the next account posting listings.