Sounds like a poor salesman. I bet he sold you NOTHING, right? You walked out of the store without buying.
Sigma and Tokina each have some good lenses, as well as some that are pretty poor. Tokina's 80-400 for example is really a dog. Also, Tokina's 28-80mm f/2.8 AT-X PRO, one of their their "best series" of lenses, was utter rubbish! I dubbed my copy "Der Flaremeister". It was utter crap, and was positioned against the Nikkor 28-70 f/2.8 AFS, which was a dead-on solid performer.
Sigma's 70-200 f/2.8 EX DG HSM OS...read the review of it at dPreview...as they say very subtly, so as not to offend AMazon.com, the company that bought dPreview...the Sigma comes up decidedly short in performance at f/2.8 compared to both Canon and Nikon 70-200 lenses. YES, the Sigma costs significantly less money than the Nikkor of the same spec, and yes, the Sigma is decidedly optically weaker.
Nikon USA gives a full, 5-year warranty on its lenses. Most other companies stand behind their lenses for one year. A person needs to compare one,specific lens model against another,specific model that it competes with. And decide what is worth paying for and what is not worth paying for. Sigma makes some good macro lenses.
However, please note: Sigma lenses are now NOT FUNCTIONING RIGHT with Nikon's new D5300 model. Look into that. Nikon had re-engineered the focusing system of the D5300, and Sigma lenses that have not been updated are now not functioning right, so there is no guarantee moving forward that Sigma lenses will work properly with future Nikon cameras. The SAME problem occurred back when the Nikon D200 came out.
http://www.dpreview.com/news/2013/12/03/sigma-issues-advisory-on-lens-compatibility-with-nikon-df
EDIT: OH, apparently the issues ALSO exists with the new Nikon Df. See the pattern?