You're right about having to do minimum amount of business to keep the account open. You also have to give them a list of "employees" who are authorized to buy on the account. So you can't just give someone the account number and have them buy on the account, they have to be on the list. That's how it is supposed to work, maybe some stores are more lenient.
Something else to keep in mind with WM, if you call ahead to check if they have an item in stock, they will look on the computer. If the computer claims to have the item, ask them to physically put their hands on it. Many times the computer will show an item in stock but there are none on the shelf.