To avoid requiring every person who happened to sell an item of tangible personal property during the year to collect or pay sales tax, most states offer an exemption from sales tax for occasional, casual, or isolated sales. This is the reason you can have a garage sale every spring and not collect sales tax. In most cases, eligibility for the exemption is based on the seller not having more than a specified number or dollar amount of sales during the year. Among the states having a sales tax, Colorado, Oklahoma and Wyoming are the only states that do not provide specific exemptions for occasional sales (with the exception of occasional sales by charitable organizations).
However, a number of states deny the exemption to any person who is normally engaged in the business of making taxable retail sales. In contrast, other states allow the exemption to retailers with respect to sales that are not made in the regular course of their business.