You can sell unlimited items to Arturo in Diamond City, which makes extremely expensive items available to you early in the campaign.
Start by defeating enemies and looting areas until you have a few hundred caps in your possession. Then head to Diamond City and speak with Arturo. Look at his wares, and indicate your wish to purchase all of his ammo of a particular type but do <strong>not</strong> finalize the deal. Instead, access your items and sell a single unit of the ammo you just bought from him. Then, sell any remainder of that same ammo type. There is a game bug that keeps a single unit of that ammo in your inventory, which you can sell to him continuously.
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