There were times when certain individual slaves were not sold, either because they appeared too sick or too old or the slaveholder was asking for too much money, but generally almost any person capable of being exploited was capable of being sold. What you do find is elderly slaves being 'freed' so that slaveholders have no obligation to support them (and states making laws against this practice) or pregnant or new mothers being leased for low rates alongside their child because they were presumed incapable of hard work.