A certificate of resale in Illinois allows you to purchase items without paying a sales tax as long as you're going to sell those items through your business. Once you've purchased the items, you price and sell them, then pay the sales tax once you've made a profit on your purchase. You receive your certificate of resale through the vendor. However if you're a buyer for a business that doesn't resell to taxable entities, like a school, you'll have to apply for a resale number that allows you to avoid paying extra taxes on those sales.
Read the Retailers' Occupation Tax Act put forth by the Illinois Department of Revenue. Check to be sure that you are not required to pay taxes through that or any other tax laws before you apply for your resale number.
Write a letter detailing why your purchases for resale should not be taxed, as per Illinois law. Explain who your clientèle are, what you purchase for resale, and your primary occupation.
Obtain a copy of your client's sales tax exemption and include this with your letter to prove that you shouldn't have to pay resale tax on untaxed items sold.
Send the letter and the proof to your regional office. Addresses are available on the Illinois Department of Revenue website.