Sell parts, weapons, and gained mothership modifiers at shops

[kouwei32]

We should be able to sell parts, weapons, as well as Nimbatus modifiers gained in the playthrough for Tritium and Unobtainium at shops or other applicable outposts, at roughly 70% of original price if applicable, and otherwise with a generated price depending on rarity, usefulness, and upgrades (for weapons). Also should have lossless buyback as QoL.

[joemama1512]

I absolutely would not do mothership modifiers as they are priceless.   But yes, selling weapons and parts does seem like a reasonable idea.  It would help if nothing else to sort out the junk weapons without having to go to the casino.