What tools do I need for online arbitrage?

A workable online-arbitrage stack is a profit-and-gating checker with built-in price history (SellerAmp or BuyBotPro) and, once you are sourcing at volume, a bulk scanner (Tactical Arbitrage) that runs supplier catalogs against Amazon automatically. Many sellers add a deal-list subscription to skip the scanning. Start manual with the first two so you learn to read the data before you pay to automate it.