Both of these answers assume U.S. law.
Craigslist has no liability per a statute commonly known as Section 230.
In a barter transaction, each party must declare as income on their tax return the fair market value of the goods or services received, by a commercially reasonable means (subject to audit), less any basis (or, in the case of accrual accounting taxpayers, at least, any cost of good sold expense deduction) applicable to goods parted with.
Under changes to the tax law effective January 1, 2018, this answer applies even if the barter involved tangible personal property of "like kind" which was previously not taxable under Section 1031 of the Internal Revenue Code.
Section 1031 continues to provide special treatment to barters involving investment real estate.