This is the 1st time we are selling a car here in Thailand and I have a important question about getting our money.

The buyer is a western foreigner who seems very sincere but since it is a nice sum of money, we still we want to be cautious.

We prefer the payment in cash because we are not sure how to determine if a bank's cashier check is legitimate.

After we arrive at the DLT, at what point in the transfer of ownership process do we exchange the cash and keys?

I would imagine it would be done when you see the name change in the book. If you are paranoid about it, get the money before signing any documentation to change the name .... I don't think you will have any issues. It's a bit strange to deal with large transactions in cash for me. Can't it be done by bank transfer? That being said, I did make the down payment on my car at Honda in cash. It felt a bit odd!

When we bought the land, my wife got a special kind of cheque from the bank. So you don't travel with a bag full of cash. And than went with the seller to the sellers bank and there the bank was booked on her account.

At any point get 50% up front once you get to the agency is how I'd handle it or meet prior and get it then and then go to the agency and it's possible the purchaser will just offer the entire amount. I use a local agent though and complete the sale at that point instead of going to the agency directly, it's worth it to me instead of dealing with the agency for the slim fee the local agent charges.. Especially if the MOT is far away.

^ H90 that's called a cashiers check and that's what the OP is concerned about where it's legitimacy is concerned. Cars can disappear where as land can't so much easier to abscond with the car before knowing the check is worthless.