Flip without leaving the city: put in a buy order just above the best one, and once it fills, a sell order just under the cheapest. The margin is the gap after the setup fee on both orders and the tax on the sale. Every item and quality, in every city, on every server.
How long an order takes to fill depends on how much of the item trades, which the Data Project does not report for every item: the item page's history shows what sold per day in each city. Prices are the latest buy and sell orders players' clients saw, each with its age; a margin that only shows with old prices may be gone.