I never keep unique weapons around unless they're part of my character's unique skillset. If my character is Explosives, you can bet your [censored] I'm selling any unique gun/energy/Unarmed/Melee weapon I see. Like you said, there's no point in holding them, and the caps you get are well worth the trade in helping you progress.
If it's useless junk like Remnants armor and the Bozar, go ahead and sell.
I never sell any unique weapons, for some raisin I store them all in a footlocker in the Lucky 38. Actually I usually end up with fully-repaired weapons of many types stashed all over, in places like the Abandoned BoS bunker, the SINK, any faction safe houses I can enter.
I have sold a few unique armors, if it isn't the type my current character would use.
Well I only get the unique weapons that fit my character. I don't go out of my way to get other ones since caps are easy to get and I don't have a use for them.
I don't as well, but I usually just encounter them along my travels and exploring, I keep forgetting that they're in the locations...I think it's just my old Fallout instincts kicking in subconsciously.