I'll start this with a less-than-definite source, but in the case of Solomon Northup (Twelve Years a Slave, the book not the movie), slaves who went unsold were typically put right back up next time for auction.

In some cases, if they sabotaged the sale (which they sometimes did) by lying about age, their health, etc...for example, things like that to get back at their masters...they would be punished if found out. Otherwise, they would likely just be put up again and again until someone made an offer that agreed with the owner.

That's what happened in Northup's account of it as a slave on the auction block, at least. And yes, his story has been accepted as an autobiography, thanks to numerous verifications. However, he lived in antebellum America, not the colonial period, just to make that clear.

In the case of 170 Gold Coast slaves imported in 1762, illness inhibited their sales. The unsold slaves were moved from Wilmington (where they landed) to Cooper's Ferry, opposite Philadelphia. Even then, 14 slaves were left under the care of a doctor while they recovered, according to the vendor, who announced it to let people know more would be coming. But again, slaves that didn't sell were sold again.

In Pennsylvania again, in colonial America, the "season had ended" when "a fine few slaves" reached Philadelphia in September. The owners had trouble selling, so the slaves were sent south, eventually going as far as North and South Carolina.

In another example, Lydon says that "McManus notes that in years when slaves glutted the New York market their owners sent them to New Jersey to be held for later sale". This, too, refers to the colonial period. McManus states that "hundreds of unsold slaves were confined in makeshift quarters at Perth Amboy where buyers could obtain them at distress prices". I'll cite McManus as well, so you know where that's from.

Sources:

Northup, Solomon. 12 Years a Slave: A True Story of Betrayal, Kidnap and Slavery. London: Hesperus, 2013. Print.

Henkin, David. Becoming America. S.l.: Mcgraw-Hill, 2014. Print.

Darold D. Wax. Africans on the Delaware: The Pennsylvania Slave Trade, 1759–1765 Pennsylvania History, Vol. 50, No. 1 (January, 1983), pp. 38-49

James G. Lydon. New York and the Slave Trade, 1700 to 1774 The William and Mary Quarterly, Third Series, Vol. 35, No. 2 (Apr., 1978), pp. 375-394

Edgar J. McManus, A History of Negro Slavery in New York (Syracuse, 1966)