If you are selling to the end user - and not to reseller - yes - you need to be registered and charge sales tax.
Register with IDOR before you make any sales or when you hire an employee. For those businesses that have sales or use tax liability in Illinois, you will receive a Certificate of Registration that must be displayed in a prominent location in the place of business to which it applies. To register your business and receive your Certificate of Registration and Illinois Business Tax number.
"Sales tax" is imposed on a seller's receipts from sales of tangible personal property for use or consumption. Tangible personal property does not include real estate, stocks, bonds, or other "paper" assets representing an interest.
Because you purchased goods to resell them - you need to provide a copy of your Certificate of Registration to suppliers - to avoid sales tax payments.
If you already paid sales taxes to suppliers - that amount should be credited on your sales tax return - thus reducing the amount you have to remit.
Please check these FAQ about IL sales tax issues and this publication. Pub-113 Retailer's Overview of Sales and Use Tax .
Let me know if you need any help.
Tax Preparer
Taxes, Immigration, Labor Relations