Was the cave of Abraham in Shechem or Hebron? Genesis 23:17–20 says:


  So Ephron’s field in Machpelah near Mamre—both the field and the cave in it, and all the trees within the borders of the field—was deeded to Abraham as his property in the presence of all the Hittites who had come to the gate of the city. Afterward Abraham buried his wife Sarah in the cave in the field of Machpelah near Mamre (which is at Hebron) in the land of Canaan. So the field and the cave in it were deeded to Abraham by the Hittites as a burial site. (NIV)


Acts 7:15–16 says:


  Then Jacob went down to Egypt, where he and our ancestors died. 
  Their bodies were brought back to Shechem and placed in the tomb that
  Abraham had bought from the sons of Hamor at Shechem for a certain sum
  of money. (NIV)


You may say that Jacob was simply buried in a different burial site. However, Genesis 50:12–14 clearly states otherwise:


  So Jacob’s sons did as their father commanded. They carried his
  body to the land of Canaan and buried it in the cave in the field of
  Machpelah near Mamre. Abraham had bought this cave and field from
  Ephron the Hittite to use as a burial place. After Joseph buried
  his father, he returned to Egypt, along with his brothers and everyone
  who had gone with him to bury his father. (NCV)


It seems that both the location and the seller is off between the books. How can these passages be reconciled?
The account in Genesis seems pretty clear that the cave of Abraham is in Hebron. Modern commentators seem to agree that Stephen was "telescoping" multiple Genesis accounts of patriarchal burials into one shorter narrative. I. Howard Marshall's commentary says, for example:


  The relation of the story of the burial to the Old Testament
  traditions is complicated. According to Acts they were all buried at
  Shechem in the tomb that Abraham had bought from the sons of Hamor. (1) According to Genesis 49: 29-32; 50:13 Jacob was buried in the cave
  of Machpelah near Hebron which Abraham had bought from Ephron the
  Hittite (Gn. 23). (2) Joseph was buried at Shechem (Jos. 24:32) in
  land which Jacob had bought from the sons of Hamor (Gn. 33:18-20). (3)
  Josephus states that Jacob's other sons (and, by implication, Jacob
  himself) were buried at Hebron (Jos., Ant. 2:199), and this
  tradition is also found in Jubilees and the Testaments of the
  Twelve Patriarchs. (4) There was a local tradition at Shechem that
  the twelve sons of Jacob were buried there. It thus appears that
  Stephen differs from the Old Testament accound in that he locates the
  tomb which Abraham bought at Shechem, not Hebron, and in that he adds
  the detail about the brothers of Joseph being buried there also.
  [F.F.] Bruce (Book, p. 149 n.39) suggests that, just as Stephen has
  telescoped the two calls of Abraham at Ur and Haran in verse 2 and the
  two divine messages in verse 7, so here he has telescoped the two
  accounts of purchases of land in Canaan. It seems probable that
  Stephen has followed a tradition, according to which not only Joseph
  (he, rather than Jacob, is perhaps meant be he died, himself in
  verse 15b) but also his brothers were buried at Shechem, and that he
  has attributed the purchase of the grave there to Abraham by including
  an allusion to the story in Genesis 23. The interest in Shechem and
  the emphasis upon it is remarkable in a speech addressed to Jews in
  Jerusalem, but they certainly could not contest the fact of Joseph's
  burial in the hated Samaritan territory. There is nothing sacrosanct
  about Judea as a place of burial; is there perhaps also a subtle
  preparation of Luke's readers for the story of the evangelism of
  Samaria (8:5-25)?


Does that make Luke a bad historian? If he was accurately reflecting Stephen's words, it makes him a good historian. Does it make Stephen a sloppy speaker? Not necessarily. This article by an apologist (Timothy W. Dunkin) defends Stephen from that charge. He claims that such telescoping was a perfectly acceptable practice for the time:


  What we see in this passage is simply that either Stephen in his
  sermon, or Luke in his recounting of the sermon, is telescoping the
  events of Abraham and Jacob together, as they are essentially similar
  subject matter, and are related chronologically and historically.
  Again, let us remember who Stephen's audience were - Hellenistic Jews
  who would to a certain degree have been Hellenised. As such, they
  would have been more receptive (and perhaps would subconsciously
  expect) a presentation in a Hellenistic style of rhetoric. It is
  doubtful that Stephen's audience would have even thought twice about
  Stephen's supposed error - they would have recognised the oratory
  device, and filled in the unspoken details themselves from their tacit
  knowledge of the stories and texts in question. This is made all the
  more likely because of the fact that most ancient cultures utilised
  high-context communication. Whereas we, in our low context society,
  feel the need to explicitly spell out exactly what we mean and leave
  little to the tacit knowledge of our audience, the ancients left much
  of the context of their words unspoken. The hearers or readers were
  expected to fill in the details from what they already knew, as
  Stephen would likely have expected his audience to do in this
  situation.
  
  Of course, this telescoping device was not solely limited to
  Hellenistic venues. Hints of it also seem to appear in the Hebrew
  scriptures. We see it somewhat in Joshua's sermon in Joshua 24, such
  as where Joshua appears to get his chronology backwards in vv. 11-13.
  The creation account in Genesis 1, where the events of the sixth day,
  especially the creation of Adam and Eve, are telescoped together and
  the much greater detail appearing in Genesis 2 is left out. Despite
  (though not necessarily contradictory to) all the emphasis that has
  been laid out about the Hellenism of Stephen's audience, we can see
  that Stephen's sermon delivery is firmly in the Hebrew tradition.
  Sermonry through the recounting of historical details and progression,
  especially as they apply to God's dealings with the nation of Israel,
  are a programmatic Hebrew method of exhortation. This methodology is
  used extensively by Moses (e.g. Deut. chs. 4, 29, 32), is the basis of
  Joshua's sermon in Joshua 24, forms the basis of Psalm 78, and is
  found elsewhere in the Hebrew scriptures. This same sort of Hebraism
  appears elsewhere in the New Testament, such as in Corinthians 10 and
  Hebrews 11.
  
  Thus, Stephen's sermon was fully in line with both the Hebraic and
  Hellenistic traditions of his audience, and his meaning would have
  been easily recognised by the members of his audience. While this
  telescoping appears to modern, low-context observers as wrong, it is
  incorrect to say that this is a "contradiction" or "error" given what
  has been said above. This only appears to be a contradiction because
  our modern minds are not conditioned to recognise and expect
  Hellenistic modes of speech.