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The following three markets are great for buying one item at a time and flipping immediately. They usually take around five minutes to purchase through a buy order and then sell within another five minutes.
LET ME REPEAT MYSELF: This market is great for buying one at a time; not 10, not 50, not 100, but just a SINGLE buy order then sell it. You get greedy and you'll end up wasting your investment.
Bottle of Elonian Wine - Featured recently on our Secret Forums for a different strategy, these also make for a quick silver. At the moment they are fluctuating around 24 silver buy order and 30 silver listing (1 silver profit).
Gold Doubloon - I put out a blog post recently about Doubloons and their buy/sell prices became annoying close, except for Gold Doubloons. These will buy at 9 silver and sell at 12, effectively earning you 1 silver profit.
Heavy Bag of Coins - OK so maybe this one will make you more than 1 silver each time, but that's a good thing! Buy at 9, sell at 13 for a 2 silver profit. Also try selling these to nearby players! You can advertise that these bags are worth up to 1 gold 50 silver each. It's too random to try buying and opening these on your own, but why not let others take that risk while you get a little profit?