Mileage and age are the main things that affect price. The more miles it has the less it’s worth. So, the van you are looking at has 100,000 more miles than you thought, or 3X more than the 52,000 you thought it had. So, it’s worth a lot less than you thought is was worth.
When it comes to shopping for a vehicle (or anything else) patience pays off. If this van isn’t the right one, be patient and keep looking. Also, don’t be afraid to offer less than asking price. Each day their van doesn’t sell gives you more room to negotiate. Be sure to tell them you’re looking at some other vans too, and still trying to decide which one will be best. Always tell them it looks like a nice van, but you’re worried about the milegae, the age, the brand, etc.
You were hoping to buy a van with 52,000 miles on it. I’d keep trying to find it. A 52,000 mile van will last many many years longer than a 150,000 mile one will.