As of yet, there is no way to sell back anything in the game. However, almost all weapons have end-game potential. Shotgun 2, for instance, is commonly run with Railgun 16 on a 5-star Killshot. Thermal lance 2 is also used a lot to create an overheat effect in the end-game. I would not even consider selling paragon or lancer, as those can be enjoyable when other builds get boring. Don't worry about selling anything because in my experience all weapons and mechs have their own unique advantage/use. 

Ok  thanks  so  much.  Yes  i  already  have  a  weird  setup  with  a  high  and  low  energy  weapon  on  my  panther.  

Hello, Pilots! We won't implement an opportunity to sell in-game stuff, but we definitely fix this issue in the future and come up with how to get rid of needless items. 

Yeah, that'll be nice.