In Chapter 6 or later, the Combat Scope Betas cost 3,000 gold, one glass shard, and one scrap of iron to make. The wandering merchant sells glass shards for 200 gold and iron scraps for 150 gold. Combat Scope Betas sell for 4,100 gold. So you will make 750 gold per scope sold. The best time to do this is chapter 6-7, before the merchant starts wandering she is always at the Waterless Bridge. In Chapter 8+, she can be at Forsaken, the Waterless Bridge, or the Righteous Tower. Get into an enemy counter, run away, and check the location to see if she is there. Time phase changes also alter her position. Repeat as desired.