Online arbitrage relies on finding differences in selling price from one retailer, to the expected selling price you think you can get from the customers on selling platform you are using. The biggest problem I find with online arbitrage is the time and effort it takes to find products suitable for arbitrage. You can get software that helps you by scanning retail websites, but you still have to do the work of reviewing the results, placing the orders to buy the merchandise, prepping it according to FBA rules, and shipping it to the FBA DC. The selling fees charged by Amazon, and the extra fees charged to use Amazon FBA get very expensive, add in any shipping, handling, and product preparation fees, and you would need to identify products where you could expect to make about 50 % gross profit margin. In other words, you need to double your money on each sale. Next, you need to find a regular supply of your arbitraged product to make the best use of Amazon FBA services. It is not a really good use of your time and effort to go thru everything, just to sell a small quantity.
Now if you want to use arbitrage to ship smaller quantities direct from the retailer to your customer, without getting FBA involved it makes more sense.
If you want to use FBA I suggest you spend your time and effort to find manufacturers or distributors to source your products from. You get continuity of supply and easy transactions.