A truck on autotrader that is a 18 Ram 2500 almost a 100K miles base truck. The add said $3,638. I emailed, it is now 30,900. Do the have to honor?

That depends.  When someone advertises a price they generally have to honor it once it has been accepted, but not when the price is an obvious mistake.  I don't know what the value of the truck would be on the open market, but if it is obvious that $3,600 was way below value (which it would be if the truck is really worth over $30,000), the seller would not be bound by it.,