Was the sale of indulgences one of the biggest issues that Martin Luther had with the Catholic Church?

Actually, Luther's problem came from his reading of Paul's
Epistle to the Romans, in which he read "the just shall live by faith." This occurred
after Luther had experienced enormous personal torment feeling that nothing that he
could do would free him from sin. His ultimate conclusion was that faith alone was
necessary for salvation; good works did not figure into the equation. True, as indicated
by the earlier post, indulgences were being sold for repairs to St. Peter's Basilica,
and Luther engaged in a lively debate with Johann Tetzel, who was pushing the sale. Most
scholars agree that Tetzel won the debate, however. To Luther, good works were worthless
in seeking salvation; indulgences were a good work, therefore they were worthless, even
though they might be for a good cause.  

Since salvation
came from faith alone, there was no need for anyone to intercede on behalf of a
penitant, neither priest nor pope. This obviated the idea that the Pope was the emissary
of Christ on earth. Luther rather argued for the "priesthood of all believers." So to
Luther, indulgences were worthless, and should not be promoted by the
church.