You can sell items to buy new ones. You just need to have obtained the weapon or costume, you don't need them all in your inventory at the same time. I sold a number of items at Jimbo's, looked back in my "Collectibles" and they were still there with the name (no ???). I then used that money to buy the 2 expensive weapons at Jimbo's shop and popped the achievement there with only about half still in my inventory. Hope this helps someone.
Just a note for those wanting the "Full Arsenal" achievement
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