Boston was staunchly antislavery and hostile to southerners who had no reason to go there other than perhaps a family tradition of attending Harvard. Philadelphia was better described as neutral ground for visiting southerners. One of the largest slaveowners in the US lived in Philadelphia while foremen ran his plantations in South Carolina. He sold most of his slaves at the biggest slave auctions in American history in the late 1850s. Just in time, eh?

(http://www.autoadmit.com/thread.php?thread_id=5541240&forum_id=2#47750088)