New Post: minimum cart total just for dealers

You would need to come up with a script to read the value of the total on the checkout and if it's less than 300, use CSS to hide the button for the next step.

You can inject the script into the page in a text module and make the module only visible to the dealer role so that it only affects them.

I'm a js hack and not much use at doing the actual script... as Dave well knows!
Rob