A few points:
(1) To be honest, the problem is not the line to get the tickets. Those lines are relatively short, even during busy times. There are even kiosks for the tickets. The problem are the lines into the Acropolis once you have the tickets. You are not saving yourself much time if you order the tickets ahead.
(2) Further, I would not buy tickets for places in Greece remotely. I've got burned several times previously. You have no idea what you are buying, if it is a legitimate vendor, if the tickets will be valid when you get there, if there are discounts (there are various discount schemes based on age, citizenship status, school status, etc., that they do not tell you up unless you ask for it), and various combination schemes with other archeological sites in Athens.
(3) If you want to obviate long lines, do not go through the main entrance, but through the entrance in the southeast corner of the Acropolis. But it is a longer hike to get to the top.
(4) Finally, if you really want to avoid long lines, get there first thing in the morning. The additional bonus is that it wont be a hot, and you'll enjoy your time at Acropolis more.
Good luck!