Taxable resale of a trailer within six months of the original purchase

​​​​​​​Four months ago, we purchased a new trailer (rated 28,000 pounds GVW) from a dealer for our business. The dealer paid tax under Internal Revenue Code Section 4051 on his sale of the trailer to us. Unfortunately, the trailer we bought is not sufficiently meeting our needs and a customer is interested in buying it. A friend told me that our sale of the trailer would be taxable, however I thought that once a trailer was sold in a taxable sale, any subsequent sale would not trigger tax.