Chapter 6 or later required Combat Scope Betas cost 3000 gold, one glass shard, and one scrap of iron to make. The wandering merchant sells glass shards and iron scraps for 200 and 150 gold respectively. Combat Scope Betas sell for 4100. Profit is 750 per scope, multiplied by 999, is 749250. 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 plus, she can be at Forsaken, the Waterless Bridge, or the Righteous Tower. Your best bet then is to get into an enemy counter, run away, and check the location to see if she is there. Time phase changes also change her position. Repeat as necessary.