Ubercart - Sale approval required before payment Durpal  6, Ubertcart 6.x-2

We are selling table space to vendors at an upcoming convention. We want to let them apply but wait for admin approval before they can pay. I've been searching for several days but cannot find a solution.

Is there a way to require admin approval of a sale before moving to the payment process? 

Edit: I have no way of telling that they are vendors until they purchase a vendor table.
There's many ways to go about this but I would use Rules &amp; custom Role called Vendor. You can specify on Drupal load if a user doesn't have a Vendor role &amp; is loading the cart/checkout page. That you can redirect them to your application form with a message prompting them to apply before checkout.

Here's an example rule:

{ "rules_check_vendor" : {
    "LABEL" : "Check Vendor",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "TAGS" : [ "test" ],
    "REQUIRES" : [ "rules" ],
    "ON" : { "init" : [] },
    "IF" : [
      { "text_matches" : {
          "text" : [ "site:current-page:path" ],
          "match" : "cart\/checkout",
          "operation" : "starts"
        }
      },
      { "NOT user_has_role" : {
          "account" : [ "site:current-user" ],
          "roles" : { "value" : { "3" : "3" } }
        }
      }
    ],
    "DO" : [
      { "drupal_message" : { "message" : "In order to become a vendor. Please contact us." } },
      { "redirect" : { "url" : "contact" } }
    ]
  }
}