Here's how to flip. You can do that same with any item, but I'd recommend cannonballs and broad-tipped bolts.
Buy 1 for 1k:
Sell it back for 1gp:
Buy 10k for the price you sold at:
Sell 10k for the price that you bought at:
Took about 5 mins, for 40k profit off 2.5m. If you have 5m, that's 80k profit. 50m - 800k profit. Etc.