Perseverance. This is the key attribute required of a salesperson. According to the Merriam-Webster dictionary, perseverance is "the continued effort to do or achieve something despite difficulties, failure or opposition".
That's the reason a salesperson is taught never to get tired of knocking doors irrespective of the number of times the doors get slammed on his/her face. The salesperson is also taught never to take a "NO" from a prospect at face value. Just because a prospect has not bought from you on your first sales call doesn't necessarily mean that s/he cannot be converted to a consumer the next time around.
This "perseverence" mantra is drilled so hard into a salesperson's head that it becomes part of his/her DNA. Though meant to be a boon, perseverence can also become a salesperson's bane. This happens when the salesperson refuses to let go of any of his/her prospects in the hope that the prospect will eventually buy at some point of time. Here, the never-say-die spirit actually makes the salesperson lose his/her ability to "qualify" a prospect as a potential buyer.
Therefore, along with perseverance, a good salesperson should also know when to let go. S/he should be able to judge precisely when a prospect needs to be qualified as "dead" and move on without expending any more time.