Merchants and commercial interests in northern colonies, such as Massachusetts and Rhode

Island, were opposed to the slave trade and did not benefit from it.

True. Most slave owning states had a vested interest in continuing slavery while others simply did not.

Explanation:

States like Massachusetts and Rhode Island were either industrialized or relied on thing other than farming for sustenance.

As slaves were not employed in these parts it was easier for people there to campaign against slavery.

Down south, the white farmers relied on slaves for unpaid labor and feared that they will lose massive amount of money and workforce if slavery was outlawed.

So vested interests did play a huge role in advocacy.