What to do if my former employer used my home improvement store account to purchase materials for customers homes but now refuses to pay the bill?

Presumably, the agreement (whether written or oral) made before or after you let your employer use your card was that he would reimburse you for all  expenses; if so, and you had not agreed in advance to simply gift the purchases to your former employer, you may sue him for breach of contract. That is, you may bring a lawsuit to recover all the money from him, pursuant to the agreement that you would lend him your card and he would repay all purchases; for amounts less than $2,500 or so, you're better off suing in small claims and acting as your own attorney; for more than that, you may wish to retain a lawyer.