I do think that you can rather freely sell low MR weapons (with the exception of Duviri weapons).
In most cases you do not need more than one weapon from a family, like with Braton - you can just keep Braton Prime, other variants can be sold after ranking (however Vandal Braton is really hard to get, so I would keep it just as a Collectible Item).
However looking long term, I think you should try to keep most of MR 7+ weapons and not sell Variant weapons (with exception of MK-1, those are worthless, just Mastary fodder).