If you are a W-2 employee, and you are required to purchase supplies out of your own pocket for use in your job, the most common way to handle this is to obtain reimbursement from your employer. Your employer can reimburse you for work-related expenses tax-free through an accountable plan, which means that you are only reimbursed for expenses you prove with receipts. It doesn't matter how or why you purchased the supplies—if you purchase supplies for work, your employer may reimburse you.
(In fact, state law might require they reimburse you, or pay for the supplies themselves. You may wish to discuss with your state labor board.)
Anything else starts to seem really strange and dodgy. For example, they might refuse to compensate you for the supplies. That's morally wrong, especially if they require the supplies, and it may be illegal. Why can't the company purchase these supplies directly and issue them to their employees? I don't understand why you would want to "Sell" the supplies to the company, that makes you a business. Most legitimate companies have conflict of interest policies that limit the ability of someone being an "employee" at the same time as being a vendor who also sells supplies to the business, since that creates a situation where the employee arranges to buy supplies from himself at an inflated price, which harms the company.
Simple reimbursement under an accountable plan is the method that would be most commonly accepted and least subject to question or abuse.
