Ye were bought with a price (τιμης ηγορασθητε). See on 1 Corinthians 6:20 for this very phrase, here repeated. Both classes (slaves and freemen) were purchased by the blood of Christ.

Become not bondservants of men (μη γινεσθε δουλο ανθρωπων). Present middle imperative of γινομα with negative μη. Literally, stop becoming slaves of men. Paul here clearly defines his opposition to human slavery as an institution which comes out so powerfully in the Epistle to Philemon. Those already free from human slavery should not become enslaved.