"Affiliate" means a person that, with respect to another person: (i) has a direct or indirect ownership interest of more than 5 percent in the other person; or (ii) is related to the other person because a third person, or a group of third persons who are affiliated with each other as defined in this subsection, holds a direct or indirect ownership interest of more than 5% in the related person.
"Marketplace" means a physical or electronic place, forum, platform, application, or other method by which a marketplace seller sells or offers to sell items.
"Marketplace facilitator" means a person who, pursuant to an agreement with an unrelated third-party marketplace seller, directly or indirectly through one or more affiliates facilitates a retail sale by an unrelated third party marketplace seller by:
"Marketplace seller" means a person that sells or offers to sell tangible personal property through a marketplace operated by an unrelated third-party marketplace facilitator.
A marketplace facilitator shall determine on a quarterly basis, ending on the last day of March, June, September, and December, whether he or she meets the threshold of either paragraph (1) or (2) of this subsection (b) for the preceding 12-month period. If the marketplace facilitator meets the threshold of either paragraph (1) or (2) for a 12-month period, he or she is considered a retailer maintaining a place of business in this State and is required to collect and remit the tax imposed under this Act and file returns for one year. At the end of that one-year period, the marketplace facilitator shall determine whether the marketplace facilitator met the threshold of either paragraph (1) or (2) during the preceding 12-month period. If the marketplace facilitator met the threshold in either paragraph (1) or (2) for the preceding 12-month period, he or she is considered a retailer maintaining a place of business in this State and is required to collect and remit the tax imposed under this Act and file returns for the subsequent year. If at the end of a one-year period a marketplace facilitator that was required to collect and remit the tax imposed under this Act determines that he or she did not meet the threshold in either paragraph (1) or (2) during the preceding 12-month period, the marketplace facilitator shall subsequently determine on a quarterly basis, ending on the last day of March, June, September, and December, whether he or she meets the threshold of either paragraph (1) or (2) for the preceding 12-month period.
35 ILCS 105/2d