The difference between Acts 20:28 and other passages that speak of Christians being redeemed or purchased seems quite striking. While some scholars may wish to attribute the putative disparity to Luke's individual view of God's salvific activity through Christ, it appears that the GNT as a whole does make a vital distinction between God's divine act of redemption and the redemptive agent, Jesus Christ.
For instance, while Luke employs περιεποιήσατο in Acts 20:28 ( a morphological form that occurs once in the GNT), other verses from the Pauline and Johannine literature use a form of ἀγοράζω:
ἠγοράσθητε γὰρ τιμῆς· δοξάσατε δὴ τὸν θεὸν ἐν τῷ σώματι ὑμῶν (1 Cor 6:20).
τιμῆς ἠγοράσθητε· μὴ γίνεσθε δοῦλοι ἀνθρώπων (1 Cor 7:23).
1 Cor 7:22 relates that "anyone in the Lord" is also "a slave of Christ," indicating that the one who purchased or redeemed the Corinthians is Christ. (Notice Paul's discussion regarding πορνεία in 1 Cor 6:19-20 as well.)
2 Pt 2:1 employs τὸν ἀγοράσαντα αὐτοὺς δεσπότην and it could fittingly refer to Christ since Peter writes elsewhere that Christians were "redeemed" (ἐλυτρώθητε) from vain conduct by means of Christ's precious blood (1 Pt 1:18-19). Compare Luke 24:21; Titus 2:14.
Finally, the Lamb bought (ἠγόρασας) persons for God out of every tribe and nation (Revelation 5:9). Maybe Rev 14:3 primarily has Christ's role in mind then, when it reports that the 144,000 have been purchased from the earth:
οἱ ἠγορασμένοι ἀπὸ τῆς γῆς.
I therefore conclude that Christ could well be the δεσπότης spoken of in 2 Pt 2:1 and Jd 4. What is more, Luke may have intended to highlight God's preeminent role in the divine act of salvation. God sent His Son to redeem obedient humankind (Gal 4:4-5); He thus purchased us by using the Lamb to redeem persons of faith (Gal 3:13).